CaltechAUTHORS: Combined
https://feeds.library.caltech.edu/people/Ott-C-D/combined.rss
A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenThu, 20 Jun 2024 19:47:40 -0700Gravitational wave burst signal from core collapse of rotating stars
https://resolver.caltech.edu/CaltechAUTHORS:DIMprd08
Year: 2008
DOI: 10.1103/PhysRevD.78.064056
We present results from detailed general relativistic simulations of stellar core collapse to a proto-neutron star, using two different microphysical nonzero-temperature nuclear equations of state as well as an approximate description of deleptonization during the collapse phase. Investigating a wide variety of rotation rates and profiles as well as masses of the progenitor stars and both equations of state, we confirm in this very general setup the recent finding that a generic gravitational wave burst signal is associated with core bounce, already known as type I in the literature. The previously suggested type II (or "multiple-bounce") waveform morphology does not occur. Despite this reduction to a single waveform type, we demonstrate that it is still possible to constrain the progenitor and postbounce rotation based on a combination of the maximum signal amplitude and the peak frequency of the emitted gravitational wave burst. Our models include to sufficient accuracy the currently known necessary physics for the collapse and bounce phase of core-collapse supernovae, yielding accurate and reliable gravitational wave signal templates for gravitational wave data analysis. In addition, we assess the possibility of nonaxisymmetric instabilities in rotating nascent proto-neutron stars. We find strong evidence that in an iron core-collapse event the postbounce core cannot reach sufficiently rapid rotation to become subject to a classical bar-mode instability. However, many of our postbounce core models exhibit sufficiently rapid and differential rotation to become subject to the recently discovered dynamical instability at low rotation rates.https://resolver.caltech.edu/CaltechAUTHORS:DIMprd08Two-dimensional multiangle, multigroup neutrino radiation-hydrodynamic simulations of postbounce supernova cores
https://resolver.caltech.edu/CaltechAUTHORS:OTTapj08
Year: 2008
DOI: 10.1086/591440
We perform axisymmetric (2D) multiangle, multigroup neutrino radiation-hydrodynamic calculations of the postbounce phase of core-collapse supernovae using a genuinely 2D discrete-ordinate (S_n) method. We follow the long-term postbounce evolution of the cores of one nonrotating and one rapidly rotating 20 M_⊙ stellar model for ~400 milliseconds from 160 to ~550 ms after bounce. We present a multidimensional analysis of the multiangle neutrino radiation fields and compare in detail with counterpart simulations carried out in the 2D multigroup flux-limited diffusion (MGFLD) approximation to neutrino transport. We find that 2D multiangle transport is superior in capturing the global and local radiation-field variations associated with rotation-induced and SASI-induced aspherical hydrodynamic configurations. In the rotating model, multiangle transport predicts much larger asymptotic neutrino flux asymmetries with pole-to-equator ratios of up to ~2.5, while MGFLD tends to sphericize the radiation fields already in the optically semi-transparent postshock regions. Along the poles, the multiangle calculation predicts a dramatic enhancement of the neutrino heating by up to a factor of 3, which alters the postbounce evolution and results in greater polar shock radii and an earlier onset of the initially rotationally weakened SASI. In the nonrotating model, differences between multiangle and MGFLD calculations remain small at early times when the postshock region does not depart significantly from spherical symmetry. At later times, however, the growing SASI leads to large-scale asymmetries and the multiangle calculation predicts up to 30% higher average integral neutrino energy deposition rates than MGFLD.https://resolver.caltech.edu/CaltechAUTHORS:OTTapj08Neutrino Signatures and the Neutrino-Driven Wind in Binary Neutron Star Mergers
https://resolver.caltech.edu/CaltechAUTHORS:DESapj09
Year: 2009
DOI: 10.1088/0004-637X/690/2/1681
We present VULCAN/2D multigroup flux-limited-diffusion radiation-hydrodynamics simulations of binary neutron star mergers, using the Shen equation of state, covering ≳ 100 ms, and starting from azimuthal-averaged two-dimensional slices obtained from three-dimensional smooth-particle-hydrodynamics simulations of Rosswog & Price for 1.4M☉ (baryonic) neutron stars with no initial spins, co-rotating spins, or counter-rotating spins. Snapshots are post-processed at 10 ms intervals with a multiangle neutrino-transport solver. We find polar-enhanced neutrino luminosities, dominated by ¯νe and "νμ" neutrinos at the peak, although νe emission may be stronger at late times. We obtain typical peak neutrino energies for νe, ¯νe, and "νμ" of ∼12, ∼16, and ∼22 MeV, respectively. The supermassive neutron star (SMNS) formed from the merger has a cooling timescale of ≾ 1 s. Charge-current neutrino reactions lead to the formation of a thermally driven bipolar wind with (M·) ∼ 10^−3 M☉ s^−1 and baryon-loading in the polar regions, preventing any production of a γ-ray burst prior to black hole formation. The large budget of rotational free energy suggests that magneto-rotational effects could produce a much-greater polar mass loss. We estimate that ≾ 10^−4 M☉ of material with an electron fraction in the range 0.1–0.2 becomes unbound during this SMNS phase as a result of neutrino heating. We present a new formalism to compute the νi ¯νi annihilation rate based on moments of the neutrino-specific intensity computed with our multiangle solver. Cumulative annihilation rates, which decay as ∼t^−1.8, decrease over our 100 ms window from a few ×1050 to ∼ 1049 erg s−1, equivalent to a few ×10^54 to ∼10^53 e−e+ pairs per second.https://resolver.caltech.edu/CaltechAUTHORS:DESapj09The gravitational-wave signature of core-collapse supernovae
https://resolver.caltech.edu/CaltechAUTHORS:20090423-082031180
Year: 2009
DOI: 10.1088/0264-9381/26/6/063001
We review the ensemble of anticipated gravitational-wave (GW) emission processes in stellar core collapse and postbounce core-collapse supernova evolution. We discuss recent progress in the modeling of these processes and summarize most recent GW signal estimates. In addition, we present new results on the GW emission from postbounce convective overturn and protoneutron star g-mode pulsations based on axisymmetric radiation-hydrodynamic calculations. Galactic core-collapse supernovae are very rare events, but within 3–5 Mpc from Earth, the rate jumps to 1 in ~2 years. Using the set of currently available theoretical gravitational waveforms, we compute upper-limit optimal signal-to-noise ratios based on current and advanced LIGO/GEO600/VIRGO noise curves for the recent SN 2008bk which exploded at ~3.9 Mpc. While initial LIGOs cannot detect GWs emitted by core-collapse events at such a distance, we find that advanced LIGO-class detectors could put significant upper limits on the GW emission strength for such events. We study the potential occurrence of the various GW emission processes in particular supernova explosion scenarios and argue that the GW signatures of neutrino-driven, magneto-rotational, and acoustically-driven core-collapse SNe may be mutually exclusive. We suggest that even initial LIGOs could distinguish these explosion mechanisms based on the detection (or non-detection) of GWs from a galactic core-collapse supernova.https://resolver.caltech.edu/CaltechAUTHORS:20090423-082031180Computational models of stellar collapse and core-collapse supernovae
https://resolver.caltech.edu/CaltechAUTHORS:20100920-110700162
Year: 2009
DOI: 10.1088/1742-6596/180/1/012022
Core-collapse supernovae are among Nature's most energetic events. They mark the end of massive star evolution and pollute the interstellar medium with the life-enabling ashes of thermonuclear burning. Despite their importance for the evolution of galaxies and life in the universe, the details of the core-collapse supernova explosion mechanism remain in the dark and pose a daunting computational challenge. We outline the multi-dimensional, multi-scale, and multi-physics nature of the core-collapse supernova problem and discuss computational strategies and requirements for its solution. Specifically, we highlight the axisymmetric (2D) radiation-MHD code VULCAN/2D and present results obtained from the first full-2D angle-dependent neutrino radiation-hydrodynamics simulations of the post-core-bounce supernova evolution. We then go on to discuss the new code Zelmani which is based on the open-source HPC Cactus framework and provides a scalable AMR approach for 3D fully general-relativistic modeling of stellar collapse, core-collapse supernovae and black hole formation on current and future massively-parallel HPC systems. We show Zelmani's scaling properties to more than 16,000 compute cores and discuss first 3D general-relativistic core-collapse results.https://resolver.caltech.edu/CaltechAUTHORS:20100920-110700162Joint searches between gravitational-wave interferometers and high-energy neutrino telescopes: science reach and analyis strategies
https://resolver.caltech.edu/CaltechAUTHORS:20091119-142513435
Year: 2009
DOI: 10.1142/S0218271809015655
Many of the astrophysical sources and violent phenomena observed in our Universe are potential emitters of gravitational waves (GWs) and high-energy neutrinos (HENs). A network of GW detectors such as LIGO and Virgo can determine the direction/time of GW bursts while the IceCube and ANTARES neutrino telescopes can also provide accurate directional information for HEN events. Requiring the consistency between both, totally independent, detection channels shall enable new searches for cosmic events arriving from potential common sources, of which many extra-galactic objects.https://resolver.caltech.edu/CaltechAUTHORS:20091119-142513435Probing the core-collapse supernova mechanism with gravitational waves
https://resolver.caltech.edu/CaltechAUTHORS:20091022-145115826
Year: 2009
DOI: 10.1088/0264-9381/26/20/204015
The mechanism of core-collapse supernova explosions must draw on the energy provided by gravitational collapse and transfer the necessary fraction to the kinetic and internal energy of the ejecta. Despite many decades of concerted theoretical effort, the detailed mechanism of core-collapse supernova explosions is still unknown, but indications are strong that multi-D processes lie at its heart. This opens up the possibility of probing the supernova mechanism with gravitational waves, carrying direct dynamical information from the supernova engine deep inside a dying massive star. I present a concise overview of the physics and primary multi-D dynamics in neutrino-driven, magnetorotational, and acoustically driven core-collapse supernova explosion scenarios. Discussing and contrasting estimates for the gravitational-wave emission characteristics of these mechanisms, I argue that their gravitational-wave signatures are clearly distinct and that the observation (or non-observation) of gravitational waves from a nearby core-collapse event could put strong constraints on the supernova mechanism.https://resolver.caltech.edu/CaltechAUTHORS:20091022-145115826A Model for Gravitational Wave Emission from Neutrino-Driven Core-Collapse Supernovae
https://resolver.caltech.edu/CaltechAUTHORS:20100107-142228258
Year: 2009
DOI: 10.1088/0004-637X/707/2/1173
Using a suite of progenitor models, neutrino luminosities, and two-dimensional simulations, we investigate the matter gravitational wave (GW) emission from postbounce phases of neutrino-driven core-collapse supernovae. These phases include prompt and steady-state convection, the standing accretion shock instability (SASI), and asymmetric explosions. For the stages before explosion, we propose a model for the source of GW emission. Downdrafts of the postshock-convection/SASI region strike the protoneutron star "surface" with large speeds and are decelerated by buoyancy forces. We find that the GW amplitude is set by the magnitude of deceleration and, by extension, the downdraft's speed and the vigor of postshock-convective/SASI motions. However, the characteristic frequencies, which evolve from ~100 Hz after bounce to ~300-400 Hz, are practically independent of these speeds (and turnover timescales). Instead, they are set by the deceleration timescale, which is in turn set by the buoyancy frequency at the lower boundary of postshock convection. Consequently, the characteristic GW frequencies are dependent upon a combination of core structure attributes, specifically the dense-matter equation of state (EOS) and details that determine the gradients at the boundary, including the accretion-rate history, the EOS at subnuclear densities, and neutrino transport. During explosion, the high frequency signal wanes and is replaced by a strong low frequency, ~10s of Hz, signal that reveals the general morphology of the explosion (i.e., prolate, oblate, or spherical). However, current and near-future GW detectors are sensitive to GW power at frequencies ≳50 Hz. Therefore, the signature of explosion will be the abrupt reduction of detectable GW emission.https://resolver.caltech.edu/CaltechAUTHORS:20100107-142228258Axisymmetric general relativistic simulations of the accretion-induced collapse of white dwarfs
https://resolver.caltech.edu/CaltechAUTHORS:20100413-153818038
Year: 2010
DOI: 10.1103/PhysRevD.81.044012
The accretion-induced collapse (AIC) of a white dwarf may lead to the formation of a protoneutron star
and a collapse-driven supernova explosion. This process represents a path alternative to thermonuclear
disruption of accreting white dwarfs in type Ia supernovae. In the AIC scenario, the supernova explosion
energy is expected to be small and the resulting transient short-lived, making it hard to detect by
electromagnetic means alone. Neutrino and gravitational-wave (GW) observations may provide crucial
information necessary to reveal a potential AIC. Motivated by the need for systematic predictions of the
GW signature of AIC, we present results from an extensive set of general-relativistic AIC simulations
using a microphysical finite-temperature equation of state and an approximate treatment of deleptonization
during collapse. Investigating a set of 114 progenitor models in axisymmetric rotational equilibrium,
with a wide range of rotational configurations, temperatures and central densities, and resulting white
dwarf masses, we extend previous Newtonian studies and find that the GW signal has a generic shape akin
to what is known as a ''type III'' signal in the literature. Despite this reduction to a single type of
waveform, we show that the emitted GWs carry information that can be used to constrain the progenitor
and the postbounce rotation. We discuss the detectability of the emitted GWs, showing that the signal-tonoise
ratio for current or next-generation interferometer detectors could be high enough to detect such
events in our Galaxy. Furthermore, we contrast the GW signals of AIC and rotating massive star iron core
collapse and find that they can be distinguished, but only if the distance to the source is known and a
detailed reconstruction of the GW time series from detector data is possible. Some of our AIC models
form massive quasi-Keplerian accretion disks after bounce. The disk mass is very sensitive to progenitor
mass and angular momentum distribution. In rapidly differentially rotating models whose precollapse
masses are significantly larger than the Chandrasekhar mass, the resulting disk mass can be as large as
0:8M. Slowly and/or uniformly rotating models that are limited to masses near the Chandrasekhar
mass produce much smaller disks or no disk at all. Finally, we find that the postbounce cores of rapidly
spinning white dwarfs can reach sufficiently rapid rotation to develop a gravitorotational bar-mode
instability. Moreover, many of our models exhibit sufficiently rapid and differential rotation to become
subject to recently discovered low-E_(rot)/│W│-type dynamical instabilities.https://resolver.caltech.edu/CaltechAUTHORS:20100413-153818038The third generation of gravitational wave observatories and their science reach
https://resolver.caltech.edu/CaltechAUTHORS:20101025-092224594
Year: 2010
DOI: 10.1088/0264-9381/27/8/084007
Large gravitational wave interferometric detectors, like Virgo and LIGO, demonstrated the capability to reach their design sensitivity, but to transform these machines into an effective observational instrument for gravitational wave astronomy a large improvement in sensitivity is required. Advanced detectors in the near future and third-generation observatories in more than one decade will open the possibility to perform gravitational wave astronomical observations from the Earth. An overview of the possible science reaches and the technological progress needed to realize a third-generation observatory are discussed in this paper. The status of the project Einstein Telescope (ET), a design study of a third-generation gravitational wave observatory, will be reported.https://resolver.caltech.edu/CaltechAUTHORS:20101025-092224594Searching for prompt signatures of nearby core-collapse supernovae by a joint analysis of neutrino and gravitational wave data
https://resolver.caltech.edu/CaltechAUTHORS:20101022-100053959
Year: 2010
DOI: 10.1088/0264-9381/27/8/084019
We discuss the science motivations and prospects for a joint analysis of gravitational wave (GW) and low-energy neutrino data to search for prompt signals from nearby supernovae (SNe). Both gravitational wave and low-energy neutrinos are expected to be produced in the innermost region of a core-collapse supernova, and a search for coincident signals would probe the processes which power a supernova explosion. It is estimated that the current generation of neutrino and gravitational wave detectors would be sensitive to galactic core-collapse supernovae, and would also be able to detect electromagnetically dark SNe. A joint GW-neutrino search would enable improvements to searches by way of lower detection thresholds, larger distance range, better live-time coverage by a network of GW and neutrino detectors, and increased significance of candidate detections. A close collaboration between the GW and neutrino communities for such a search will thus go far toward realizing a much sought-after astrophysics goal of detecting the next nearby supernova.https://resolver.caltech.edu/CaltechAUTHORS:20101022-100053959Equation of state effects in black hole–neutron star mergers
https://resolver.caltech.edu/CaltechAUTHORS:20100602-153604354
Year: 2010
DOI: 10.1088/0264-9381/27/11/114106
The merger dynamics of a black hole–neutron star (BHNS) binary is influenced by the neutron star equation of state (EoS) through the latter's effect on the neutron star's radius and on the character of the mass transfer onto the black hole. We study these effects by simulating a number of BHNS binaries in full general relativity using a mixed pseudospectral/finite difference code. We consider several models of the neutron star matter EoS, including Γ = 2 and Γ = 2.75 polytropes and the nuclear-theory-based Shen EoS. For models using the Shen EoS, we consider two limits for the evolution of the composition: source-free advection and instantaneous β-equilibrium. To focus on EoS effects, we fix the mass ratio to 3:1 and the initial aligned black hole spin to a/m = 0.5 for all models. We confirm earlier studies which found that more compact stars create a stronger gravitational wave signal but a smaller postmerger accretion disk. We also vary the EoS while holding the compaction fixed. All mergers are qualitatively similar, but we find signatures of the EoS in the waveform and in the tail and disk structures.https://resolver.caltech.edu/CaltechAUTHORS:20100602-153604354Microphysics in Computational Relativistic Astrophysics - MICRA2009, Niels Bohr International Academy, Copenhagen, 24–28 August 2009
https://resolver.caltech.edu/CaltechAUTHORS:20110324-105956902
Year: 2010
DOI: 10.1088/0264-9381/27/11/110302
This special section is devoted to the workshop `Microphysics in Computational Relativistic Astrophysics - MICRA2009', which took place at the Niels Bohr International Academy, in Copenhagen, 24–28 August 2009.https://resolver.caltech.edu/CaltechAUTHORS:20110324-105956902A new open-source code for spherically symmetric stellar collapse to neutron stars and black holes
https://resolver.caltech.edu/CaltechAUTHORS:20100602-140548512
Year: 2010
DOI: 10.1088/0264-9381/27/11/114103
We present the new open-source spherically symmetric general-relativistic (GR) hydrodynamics code GR1D. It is based on the Eulerian formulation of GR hydrodynamics (GRHD) put forth by Romero–Ibáñez–Gourgoulhon and employs radial-gauge, polar-slicing coordinates in which the 3+1 equations simplify substantially. We discretize the GRHD equations with a finite-volume scheme, employing piecewise-parabolic reconstruction and an approximate Riemann solver. GR1D is intended for the simulation of stellar collapse to neutron stars and black holes and will also serve as a testbed for modeling technology to be incorporated in multi-D GR codes. Its GRHD part is coupled to various finite-temperature microphysical equations of state in tabulated form that we make available with GR1D. An approximate deleptonization scheme for the collapse phase and a neutrino-leakage/heating scheme for the postbounce epoch are included and described. We also derive the equations for effective rotation in 1D and implement them in GR1D. We present an array of standard test calculations and also show how simple analytic equations of state in combination with presupernova models from stellar evolutionary calculations can be used to study qualitative aspects of black hole formation in failing rotating core-collapse supernovae. In addition, we present a simulation with microphysical equations of state and neutrino leakage/heating of a failing core-collapse supernova and black hole formation in a presupernova model of a 40 M_⊙ zero-age main-sequence star. We find good agreement on the time of black hole formation (within 20%) and last stable protoneutron star mass (within 10%) with predictions from simulations with full Boltzmann neutrino radiation hydrodynamics.https://resolver.caltech.edu/CaltechAUTHORS:20100602-140548512Studies of Stellar Collapse and Black Hole Formation with the Open-Source Code GR1D
https://resolver.caltech.edu/CaltechAUTHORS:20101012-101316082
Year: 2010
DOI: 10.1063/1.3485130
We discuss results from simulations of black hole formation in failing core-collapse supernovae performed with the code GR1D, a new open-source Eulerian spherically-symmetric general-relativistic hydrodynamics code. GR1D includes rotation in an approximate way (1.5D) comes with multiple finite-temperature nuclear equations of state (EOS), and treats neutrinos in the post-core-bounce phase via a 3-flavor leakage scheme and a heating prescription. We chose the favored K_0 = 220 MeV-variant of the Lattimer & Swesty (1990) EOS and present collapse calculations using the progenitor models of Limongi & Chieffi (2006). We show that there is no direct (or "prompt") black hole formation in the collapse of ordinary massive stars (8M_☉ ≲ M_(ZAMS) ≲ 100 M_☉) present first results from black hole formation simulations that include rotation.https://resolver.caltech.edu/CaltechAUTHORS:20101012-101316082The Einstein Telescope: a third-generation gravitational wave observatory
https://resolver.caltech.edu/CaltechAUTHORS:20101012-101244573
Year: 2010
DOI: 10.1088/0264-9381/27/19/194002
Advanced gravitational wave interferometers, currently under realization, will soon permit the detection of gravitational waves from astronomical sources. To open the era of precision gravitational wave astronomy, a further substantial improvement in sensitivity is required. The future space-based Laser Interferometer Space Antenna and the third-generation ground-based observatory Einstein Telescope (ET) promise to achieve the required sensitivity improvements in frequency ranges. The vastly improved sensitivity of the third generation of gravitational wave observatories could permit detailed measurements of the sources' physical parameters and could complement, in a multi-messenger approach, the observation of signals emitted by cosmological sources obtained through other kinds of telescopes. This paper describes the progress of the ET project which is currently in its design study phase.https://resolver.caltech.edu/CaltechAUTHORS:20101012-101244573Theoretical support for the hydrodynamic mechanism of pulsar kicks
https://resolver.caltech.edu/CaltechAUTHORS:20110520-151156536
Year: 2010
DOI: 10.1103/PhysRevD.82.103016
The collapse of a massive star's core, followed by a neutrino-driven, asymmetric supernova explosion, can naturally lead to pulsar recoils and neutron star kicks. Here, we present a two-dimensional, radiation-hydrodynamic simulation in which core collapse leads to significant acceleration of a fully formed, nascent neutron star via an induced, neutrino-driven explosion. During the explosion, an ~10% anisotropy in the low-mass, high-velocity ejecta leads to recoil of the high-mass neutron star. At the end of our simulation, the neutron star has achieved a velocity of ~150 km s^(-1) and is accelerating at ~350 km s^(-2), but has yet to reach the ballistic regime. The recoil is due almost entirely to hydrodynamical processes, with anisotropic neutrino emission contributing less than 2% to the overall kick magnitude. Since the observed distribution of neutron star kick velocities peaks at ~300–400 km s^(-1), recoil due to anisotropic core-collapse supernovae provides a natural, nonexotic mechanism with which to obtain neutron star kicks.https://resolver.caltech.edu/CaltechAUTHORS:20110520-151156536Helium ignition on accreting neutron stars with a new triple-α reaction rate
https://resolver.caltech.edu/CaltechAUTHORS:20110112-090206619
Year: 2010
DOI: 10.1088/0004-637X/725/1/309
We investigate the effect of a new triple-α reaction rate from Ogata et al. on helium ignition conditions on accreting
neutron stars and on the properties of the subsequent type I X-ray burst. We find that the new rate leads to
significantly lower ignition column density for accreting neutron stars at low accretion rates. We compare the
results of our ignition models for a pure helium accretor to observations of bursts in ultracompact X-ray binaries
(UCXBs), which are believed to have nearly pure helium donors. For ^._m > 0.001 ^._m_(Edd), the new triple-α reaction
rate from Ogata et al. predicts a maximum helium ignition column of ~3 × 10^9 g cm^(−2), corresponding to a burst
energy of ~4 × 10^(40) erg. For ^._m ~ 0.01 ^._m_(Edd) at which intermediate long bursts occur, the predicted burst energies
are at least a factor of 10 too low to explain the observed energies of such bursts in UCXBs. This finding adds to the
doubts cast on the triple-α reaction rate of Ogata et al. by the low-mass stellar evolution results of Dotter & Paxton.https://resolver.caltech.edu/CaltechAUTHORS:20110112-090206619New Aspects and Boundary Conditions of Core-Collapse Supernova Theory
https://resolver.caltech.edu/CaltechAUTHORS:20120828-094426241
Year: 2011
DOI: 10.48550/arXiv.1111.6282
Core-collapse supernovae are among Nature's grandest explosions. They are powered by
the energy released in gravitational collapse and include a rich set of physical phenomena
involving all fundamental forces and many branches of physics and astrophysics. We
summarize the current state of core-collapse supernova theory and discuss the current set
of candidate explosion mechanisms under scrutiny as core-collapse supernova modeling is
moving towards self-consistent three-dimensional simulations. Recent work in nuclear theory
and neutron star mass and radius measurements are providing new constraints for the
nuclear equation of state. We discuss these new developments and their impact on corecollapse
supernova modeling. Neutrino-neutrino forward scattering in the central regions
of core-collapse supernovae can lead to collective neutrino flavor oscillations that result in
swaps of electron and heavy-lepton neutrino spectra. We review the rapid progress that
is being made in understanding these collective oscillations and their potential impact on
the core-collapse supernova explosion mechanism.https://resolver.caltech.edu/CaltechAUTHORS:20120828-094426241Results From Core-Collapse Simulations with Multi-Dimensional, Multi-Angle Neutrino Transport
https://resolver.caltech.edu/CaltechAUTHORS:20110314-161839446
Year: 2011
DOI: 10.1088/0004-637X/728/1/8
We present new results from the only two-dimensional multi-group, multi-angle calculations of core-collapse
supernova evolution. The first set of results from these calculations was published in 2008 by Ott et al. We have
followed a nonrotating and a rapidly rotating 20M_⊙ model for ~400 ms after bounce. We show that the radiation
fields vary much less with angle than the matter quantities in the region of net neutrino heating. This happens because
most neutrinos are emitted from inner radiative regions and because the specific intensity is an integral over sources
from many angles at depth. The latter effect can only be captured by multi-angle transport. We then compute the
phase relationship between dipolar oscillations in the shock radius and in matter and radiation quantities throughout
the post-shock region. We demonstrate a connection between variations in neutrino flux and the hydrodynamical
shock oscillations, and use a variant of the Rayleigh test to estimate the detectability of these neutrino fluctuations
in IceCube and Super-Kamiokande. Neglecting flavor oscillations, fluctuations in our nonrotating model would be
detectable to ~10 kpc in IceCube, and a detailed power spectrum could be measured out to ~5 kpc. These distances
are considerably lower in our rapidly rotating model or with significant flavor oscillations. Finally, we measure the
impact of rapid rotation on detectable neutrino signals. Our rapidly rotating model has strong, species-dependent
asymmetries in both its peak neutrino flux and its light curves. The peak flux and decline rate show pole–equator
ratios of up to ~3 and ~2, respectively.https://resolver.caltech.edu/CaltechAUTHORS:20110314-161839446Gravitational wave extraction in simulations of rotating stellar core collapse
https://resolver.caltech.edu/CaltechAUTHORS:20110328-113942831
Year: 2011
DOI: 10.1103/PhysRevD.83.064008
We perform simulations of general relativistic rotating stellar core collapse and compute the gravitational waves (GWs) emitted in the core-bounce phase of three representative models via multiple techniques. The simplest technique, the quadrupole formula (QF), estimates the GW content in the spacetime from the mass-quadrupole tensor only. It is strictly valid only in the weak-field and slow-motion approximation. For the first time, we apply GW extraction methods in core collapse that are fully curvature based and valid for strongly radiating and highly relativistic sources. These techniques are not restricted to weak-field and slow-motion assumptions. We employ three extraction methods computing (i) the Newman-Penrose (NP) scalar Ψ_4, (ii) Regge-Wheeler-Zerilli-Moncrief master functions, and (iii) Cauchy-characteristic extraction (CCE) allowing for the extraction of GWs at future null infinity, where the spacetime is asymptotically flat and the GW content is unambiguously defined. The latter technique is the only one not suffering from residual gauge and finite-radius effects. All curvature-based methods suffer from strong nonlinear drifts. We employ the fixed-frequency integration technique as a high-pass waveform filter. Using the CCE results as a benchmark, we find that finite-radius NP extraction yields results that agree nearly perfectly in phase, but differ in amplitude by ~1%–7% at core bounce, depending on the model. Regge-Wheeler-Zerilli-Moncrief waveforms, while, in general, agreeing in phase, contain spurious high-frequency noise of comparable amplitudes to those of the relatively weak GWs emitted in core collapse. We also find remarkably good agreement of the waveforms obtained from the QF with those obtained from CCE. The results from QF agree very well in phase and systematically underpredict peak amplitudes by ~5%–11%, which is comparable to the NP results and is certainly within the uncertainties associated with core collapse physics.https://resolver.caltech.edu/CaltechAUTHORS:20110328-113942831Black Hole Formation in Failing Core-Collapse Supernovae
https://resolver.caltech.edu/CaltechAUTHORS:20110418-151949080
Year: 2011
DOI: 10.1088/0004-637X/730/2/70
We present results of a systematic study of failing core-collapse supernovae and the formation of stellar-mass black holes (BHs). Using our open-source general-relativistic 1.5D code GR1D equipped with a three-species neutrino leakage/heating scheme and over 100 presupernova models, we study the effects of the choice of nuclear equation
of state (EOS), zero-age main sequence (ZAMS) mass and metallicity, rotation, and mass-loss prescription on
BH formation. We find that the outcome, for a given EOS, can be estimated, to first order, by a single parameter,
the compactness of the stellar core at bounce. By comparing protoneutron star (PNS) structure at the onset
of gravitational instability with solutions of the Tolman–Oppenheimer–Volkof equations, we find that thermal
pressure support in the outer PNS core is responsible for raising the maximum PNS mass by up to 25% above the
cold NS value. By artificially increasing neutrino heating, we find the critical neutrino heating efficiency required
for exploding a given progenitor structure and connect these findings with ZAMS conditions, establishing, albeit
approximately, for the first time based on actual collapse simulations, the mapping between ZAMS parameters and
the outcome of core collapse. We also study the effect of progenitor rotation and find that the dimensionless spin of
nascent BHs may be robustly limited below a* = Jc/GM^2 = 1 by the appearance of nonaxisymmetric rotational
instabilities.https://resolver.caltech.edu/CaltechAUTHORS:20110418-151949080Long gravitational-wave transients and associated detection strategies for a network of terrestrial interferometers
https://resolver.caltech.edu/CaltechAUTHORS:20110505-113830317
Year: 2011
DOI: 10.1103/PhysRevD.83.083004
Searches for gravitational waves (GWs) traditionally focus on persistent sources (e.g., pulsars or the stochastic background) or on transients sources (e.g., compact binary inspirals or core-collapse supernovae), which last for time scales of milliseconds to seconds. We explore the possibility of long GW transients with unknown waveforms lasting from many seconds to weeks. We propose a novel analysis technique to bridge the gap between short O(s) "burst" analyses and persistent stochastic analyses. Our technique utilizes frequency-time maps of GW strain cross power between two spatially separated terrestrial GW detectors. The application of our cross power statistic to searches for GW transients is framed as a pattern recognition problem, and we discuss several pattern-recognition techniques. We demonstrate these techniques by recovering simulated GW signals in simulated detector noise. We also recover environmental noise artifacts, thereby demonstrating a novel technique for the identification of such artifacts in GW interferometers. We compare the efficiency of this framework to other techniques such as matched filtering.https://resolver.caltech.edu/CaltechAUTHORS:20110505-113830317Dynamics and Gravitational Wave Signature of Collapsar Formation
https://resolver.caltech.edu/CaltechAUTHORS:20110517-152215984
Year: 2011
DOI: 10.1103/PhysRevLett.106.161103
We perform 3+1 general relativistic simulations of rotating core collapse in the context of the collapsar model for long gamma-ray bursts. We employ a realistic progenitor, rotation based on results of stellar evolution calculations, and a simplified equation of state. Our simulations track self-consistently collapse, bounce, the postbounce phase, black hole formation, and the subsequent early hyperaccretion phase. We extract gravitational waves from the spacetime curvature and identify a unique gravitational wave signature associated with the early phase of collapsar formation.https://resolver.caltech.edu/CaltechAUTHORS:20110517-152215984Sensitivity studies for third-generation gravitational wave observatories
https://resolver.caltech.edu/CaltechAUTHORS:20110512-085754412
Year: 2011
DOI: 10.1088/0264-9381/28/9/094013
Advanced gravitational wave detectors, currently under construction, are expected to directly observe gravitational wave signals of astrophysical origin. The Einstein Telescope (ET), a third-generation gravitational wave detector, has been proposed in order to fully open up the emerging field of gravitational wave astronomy. In this paper we describe sensitivity models for ET and investigate potential limits imposed by fundamental noise sources. A special focus is set on evaluating the frequency band below 10 Hz where a complex mixture of seismic, gravity gradient, suspension thermal and radiation pressure noise dominates. We develop the most accurate sensitivity model, referred to as ET-D, for a third-generation detector so far, including the most relevant fundamental noise contributions.https://resolver.caltech.edu/CaltechAUTHORS:20110512-085754412Search for Gravitational Wave Bursts from Six Magnetars
https://resolver.caltech.edu/CaltechAUTHORS:20110826-142324216
Year: 2011
DOI: 10.1088/2041-8205/734/2/L35
Soft gamma repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are thought to be magnetars: neutron stars powered by extreme magnetic fields. These rare objects are characterized by repeated and sometimes spectacular gamma-ray bursts. The burst mechanism might involve crustal fractures and excitation of non-radial modes which would emit gravitational waves (GWs). We present the results of a search for GW bursts from six galactic magnetars that is sensitive to neutron star f-modes, thought to be the most efficient GW emitting oscillatory modes in compact stars. One of them, SGR 0501+4516, is likely ~1 kpc from Earth, an order of magnitude closer than magnetars targeted in previous GW searches. A second, AXP 1E 1547.0–5408, gave a burst with an estimated isotropic energy >10^(44) erg which is comparable to the giant flares. We find no evidence of GWs associated with a sample of 1279 electromagnetic triggers from six magnetars occurring between 2006 November and 2009 June, in GW data from the LIGO, Virgo, and GEO600 detectors. Our lowest model-dependent GW emission energy upper limits for band- and time-limited white noise bursts in the detector sensitive band, and for f-mode ringdowns (at 1090 Hz), are 3.0 × 10^(44) d^2_1 erg and 1.4 × 10^(47) d^2_1 erg, respectively, where d_1 = ^(d0501)_(1kpc) and d_ (0501) is the distance to SGR 0501+4516. These limits on GW emission from f-modes are an order of magnitude lower than any previous, and approach the range of electromagnetic energies seen in SGR giant flares for the first time.https://resolver.caltech.edu/CaltechAUTHORS:20110826-142324216Supernova Fallback onto Magnetars and Propeller-powered Supernovae
https://resolver.caltech.edu/CaltechAUTHORS:20110902-074611505
Year: 2011
DOI: 10.1088/0004-637X/736/2/108
We explore fallback accretion onto newly born magnetars during the supernova of massive stars. Strong magnetic fields (~10^(15) G) and short spin periods (~1-10 ms) have an important influence on how the magnetar interacts with the infalling material. At long spin periods, weak magnetic fields, and high accretion rates, sufficient material is accreted to form a black hole, as is commonly found for massive progenitor stars. When B ≾ 5 × 10^(14) G, accretion causes the magnetar to spin sufficiently rapidly to deform triaxially and produces gravitational waves, but only for ≈50-200 s until it collapses to a black hole. Conversely, at short spin periods, strong magnetic fields, and low accretion rates, the magnetar is in the "propeller regime" and avoids becoming a black hole by expelling incoming material. This process spins down the magnetar, so that gravitational waves are only expected if the initial protoneutron star is spinning rapidly. Even when the magnetar survives, it accretes at least ≈0.3 M_☉, so we expect magnetars born within these types of environments to be more massive than the 1.4 M_☉ typically associated with neutron stars. The propeller mechanism converts the ~10^(52)erg of spin energy in the magnetar into the kinetic energy of an outflow, which shock heats the outgoing supernova ejecta during the first ~10-30 s. For a small ~5 M_☉ hydrogen-poor envelope, this energy creates a brighter, faster evolving supernova with high ejecta velocities ~(1-3) × 10^4 km s^(–1) and may appear as a broad-lined Type Ib/c supernova. For a large ≳ 10 M_☉ hydrogen-rich envelope, the result is a bright Type IIP supernova with a plateau luminosity of ≳ 10^(43)erg s^(–1) lasting for a timescale of ~60-80 days.https://resolver.caltech.edu/CaltechAUTHORS:20110902-074611505Beating the Spin-down Limit on Gravitational Wave Emission from the Vela Pulsar
https://resolver.caltech.edu/CaltechAUTHORS:20110909-103003031
Year: 2011
DOI: 10.1088/0004-637X/737/2/93
We present direct upper limits on continuous gravitational wave emission from the Vela pulsar using data from the Virgo detector's second science run. These upper limits have been obtained using three independent methods that assume the gravitational wave emission follows the radio timing. Two of the methods produce frequentist upper limits for an assumed known orientation of the star's spin axis and value of the wave polarization angle of, respectively, 1.9 × 10^(–24) and 2.2 × 10^(–24), with 95% confidence. The third method, under the same hypothesis, produces a Bayesian upper limit of 2.1 × 10^(–24), with 95% degree of belief. These limits are below the indirect spin-down limit of 3.3 × 10^(–24) for the Vela pulsar, defined by the energy loss rate inferred from observed decrease in Vela's spin frequency, and correspond to a limit on the star ellipticity of ~10^(–3). Slightly less stringent results, but still well below the spin-down limit, are obtained assuming the star's spin axis inclination and the wave polarization angles are unknown.https://resolver.caltech.edu/CaltechAUTHORS:20110909-103003031Collisions of unequal mass black holes and the point particle limit
https://resolver.caltech.edu/CaltechAUTHORS:20111223-082957976
Year: 2011
DOI: 10.1103/PhysRevD.84.084038
Numerical relativity has seen incredible progress in the last years, and is being applied with success to a variety of physical phenomena, from gravitational wave research and relativistic astrophysics to cosmology and high-energy physics. Here we probe the limits of current numerical setups, by studying collisions of unequal mass, nonrotating black holes of mass ratios up to 1∶100 and making contact with a classical calculation in general relativity: the infall of a pointlike particle into a massive black hole. Our results agree well with the predictions coming from linearized calculations of the infall of pointlike particles into nonrotating black holes. In particular, in the limit that one hole is much smaller than the other, and the infall starts from an infinite initial separation, we recover the point-particle limit. Thus, numerical relativity is able to bridge the gap between fully nonlinear dynamics and linearized approximations, which may have important applications. Finally, we also comment on the "spurious" radiation content in the initial data and the linearized predictions.https://resolver.caltech.edu/CaltechAUTHORS:20111223-082957976New open-source approaches to the modeling of stellar collapse
and the formation of black holes
https://resolver.caltech.edu/CaltechAUTHORS:20111207-141431873
Year: 2011
DOI: 10.1007/s10509-010-0553-1
We present new approaches to the simulation
of stellar collapse, the formation of black holes, and explosive
core-collapse supernova nucleosynthesis that build
upon open-source codes and microphysics. We discuss the
new spherically-symmetric general-relativistic (GR) collapse
code GR1D that is endowed with an approximate 1.5D
treatment of rotation, comes with multiple nuclear equations
of state, and handles neutrinos with a multi-species leakage
scheme. Results from a first set of spinning black hole formation
simulations are presented. We go on to discuss the
derivative code GR1D + N which is tuned for calculations
of explosive nucleosynthesis and includes a NSE/non-NSE
equation of state treatment, and a nuclear reaction network.
We present sample results showing GR1D+N's performance
in reproducing previous results with thermal-bomb-driven
explosions. Finally, we introduce the 3 + 1 GR Zelmani
core collapse simulation package and present first results
obtained in its application to the 3D modeling of failing
core-collapse supernovae.https://resolver.caltech.edu/CaltechAUTHORS:20111207-141431873A gravitational wave observatory operating beyond the quantum shot-noise limit
https://resolver.caltech.edu/CaltechAUTHORS:20180611-155020083
Year: 2011
DOI: 10.1038/NPHYS2083
Around the globe several observatories are seeking the first direct detection of gravitational waves (GWs). These waves are predicted by Einstein's general theory of relativity and are generated, for example, by black-hole binary systems. Present GW detectors are Michelson-type kilometre-scale laser interferometers measuring the distance changes between mirrors suspended in vacuum. The sensitivity of these detectors at frequencies above several hundred hertz is limited by the vacuum (zero-point) fluctuations of the electromagnetic field. A quantum technology—the injection of squeezed light—offers a solution to this problem. Here we demonstrate the squeezed-light enhancement of GEO 600, which will be the GW observatory operated by the LIGO Scientific Collaboration in its search for GWs for the next 3–4 years. GEO 600 now operates with its best ever sensitivity, which proves the usefulness of quantum entanglement and the qualification of squeezed light as a key technology for future GW astronomy.https://resolver.caltech.edu/CaltechAUTHORS:20180611-155020083All-sky search for periodic gravitational waves in the full S5 LIGO data
https://resolver.caltech.edu/CaltechAUTHORS:20120206-151908677
Year: 2012
DOI: 10.1103/PhysRevD.85.022001
We report on an all-sky search for periodic gravitational waves in the frequency band 50–800 Hz and with the frequency time derivative in the range of 0 through -6×10^(-9) Hz/s. Such a signal could be produced by a nearby spinning and slightly nonaxisymmetric isolated neutron star in our Galaxy. After recent improvements in the search program that yielded a 10× increase in computational efficiency, we have searched in two years of data collected during LIGO's fifth science run and have obtained the most sensitive all-sky upper limits on gravitational-wave strain to date. Near 150 Hz our upper limit on worst-case linearly polarized strain amplitude h_0 is 1×10^(-24), while at the high end of our frequency range we achieve a worst-case upper limit of 3.8×10^(-24) for all polarizations and sky locations. These results constitute a factor of 2 improvement upon previously published data. A new detection pipeline utilizing a loosely coherent algorithm was able to follow up weaker outliers, increasing the volume of space where signals can be detected by a factor of 10, but has not revealed any gravitational-wave signals. The pipeline has been tested for robustness with respect to deviations from the model of an isolated neutron star, such as caused by a low-mass or long-period binary companion.https://resolver.caltech.edu/CaltechAUTHORS:20120206-151908677Implementation and testing of the first prompt search
for gravitational wave transients with electromagnetic counterparts
https://resolver.caltech.edu/CaltechAUTHORS:20120619-081035905
Year: 2012
DOI: 10.1051/0004-6361/201118219
Aims. A transient astrophysical event observed in both gravitational wave (GW) and electromagnetic (EM) channels would yield rich scientific rewards. A first program initiating EM follow-ups to possible transient GW events has been developed and exercised by the LIGO and Virgo community in association with several partners. In this paper, we describe and evaluate the methods used to promptly identify and localize GW event candidates and to request images of targeted sky locations.
Methods. During two observing periods (Dec. 17, 2009 to Jan. 8, 2010 and Sep. 2 to Oct. 20, 2010), a low-latency analysis pipeline was used to identify GW event candidates and to reconstruct maps of possible sky locations. A catalog of nearby galaxies and Milky Way globular clusters was used to select the most promising sky positions to be imaged, and this directional information was delivered to EM observatories with time lags of about thirty minutes. A Monte Carlo simulation has been used to evaluate the low-latency GW pipeline's ability to reconstruct source positions correctly.
Results. For signals near the detection threshold, our low-latency algorithms often localized simulated GW burst signals to tens of square degrees, while neutron star/neutron star inspirals and neutron star/black hole inspirals were localized to a few hundred square degrees. Localization precision improves for moderately stronger signals. The correct sky location of signals well above threshold and originating from nearby galaxies may be observed with ~50% or better probability with a few pointings of wide-field telescopes.https://resolver.caltech.edu/CaltechAUTHORS:20120619-081035905Role of collective neutrino flavor oscillations in core-collapse supernova shock revival
https://resolver.caltech.edu/CaltechAUTHORS:20120402-102559744
Year: 2012
DOI: 10.1103/PhysRevD.85.065008
We explore the effects of collective neutrino flavor oscillations due to neutrino-neutrino interactions on the neutrino heating behind a stalled core-collapse supernova shock. We carry out axisymmetric (two-dimensional) radiation-hydrodynamic core-collapse supernova simulations, tracking the first 400 ms of the post-core-bounce evolution in 11.2-M_⊙ and 15-M_⊙ progenitor stars. Using inputs from these two-dimensional simulations, we perform neutrino flavor oscillation calculations in multienergy single-angle and multiangle single-energy approximations. Our results show that flavor conversions do not set in until close to or outside the stalled shock, enhancing heating by not more than a few percent in the most optimistic case. Consequently, we conclude that the postbounce preexplosion dynamics of standard core-collapse supernovae remains unaffected by neutrino oscillations. Multiangle effects in regions of high electron density can further inhibit collective oscillations, strengthening our conclusion.https://resolver.caltech.edu/CaltechAUTHORS:20120402-102559744A New Monte Carlo Method for Time-Dependent Neutrino Radiation Transport
https://resolver.caltech.edu/CaltechAUTHORS:20120326-075052193
Year: 2012
DOI: 10.48550/arXiv.1203.2915
Monte Carlo approaches to radiation transport have several attractive properties compared to deterministic
methods. These include simplicity of implementation, high accuracy, and good parallel scaling. Moreover,
Monte Carlo methods can handle complicated geometries and are relatively easy to extend to multiple spatial
dimensions, which makes them particularly interesting in modeling complex multi-dimensional astrophysical
phenomena such as core-collapse supernovae. The aim of this paper is to explore Monte Carlo methods for
modeling neutrino transport in core-collapse supernovae. We generalize the implicit Monte Carlo photon transport
scheme of Fleck & Cummings and gray discrete-diffusion scheme of Densmore et al. to energy-, time-,
and velocity-dependent neutrino transport. Using our 1D spherically-symmetric implementation, we show that,
similar to the photon transport case, the implicit scheme enables significantly larger timesteps compared with
explicit time discretization, without sacrificing accuracy, while the discrete-diffusion method leads to significant
speed-ups at high optical depth. Our results suggest that a combination of spectral, velocity-dependent,
implicit Monte Carlo and discrete-diffusion Monte Carlo methods represents an attractive approach for use in
neutrino radiation-hydrodynamics simulations of core-collapse supernovae. Our velocity-dependent scheme
can easily be adapted to photon transport.https://resolver.caltech.edu/CaltechAUTHORS:20120326-075052193Search for gravitational waves from low mass compact binary coalescence in LIGO's sixth science run and Virgo's science runs 2 and 3
https://resolver.caltech.edu/CaltechAUTHORS:20120518-143548930
Year: 2012
DOI: 10.1103/PhysRevD.85.082002
We report on a search for gravitational waves from coalescing compact binaries using LIGO and Virgo observations between July 7, 2009, and October 20, 2010. We searched for signals from binaries with total mass between 2 and 25M_⊙; this includes binary neutron stars, binary black holes, and binaries consisting of a black hole and neutron star. The detectors were sensitive to systems up to 40 Mpc distant for binary neutron stars, and further for higher mass systems. No gravitational-wave signals were detected. We report upper limits on the rate of compact binary coalescence as a function of total mass, including the results from previous LIGO and Virgo observations. The cumulative 90% confidence rate upper limits of the binary coalescence of binary neutron star, neutron star-black hole, and binary black hole systems are 1.3×10^(-4), 3.1×10^(-5), and 6.4×10^(-6) Mpc^(-3) yr^(-1), respectively. These upper limits are up to a factor 1.4 lower than previously derived limits. We also report on results from a blind injection challenge.https://resolver.caltech.edu/CaltechAUTHORS:20120518-143548930First low-latency LIGO+Virgo search for binary inspirals and their electromagnetic counterparts
https://resolver.caltech.edu/CaltechAUTHORS:20120625-071919848
Year: 2012
DOI: 10.1051/0004-6361/201218860
Aims. The detection and measurement of gravitational-waves from coalescing neutron-star binary systems is an important science goal for ground-based gravitational-wave detectors. In addition to emitting gravitational-waves at frequencies that span the most sensitive bands of the LIGO and Virgo detectors, these sources are also amongst the most likely to produce an electromagnetic counterpart to the gravitational-wave emission. A joint detection of the gravitational-wave and electromagnetic signals would provide a powerful new probe for astronomy.
Methods. During the period between September 19 and October 20, 2010, the first low-latency search for gravitational-waves from binary inspirals in LIGO and Virgo data was conducted. The resulting triggers were sent to electromagnetic observatories for followup. We describe the generation and processing of the low-latency gravitational-wave triggers. The results of the electromagnetic image analysis will be described elsewhere.
Results. Over the course of the science run, three gravitational-wave triggers passed all of the low-latency selection cuts. Of these, one was followed up by several of our observational partners. Analysis of the gravitational-wave data leads to an estimated false alarm rate of once every 6.4 days, falling far short of the requirement for a detection based solely on gravitational-wave data.https://resolver.caltech.edu/CaltechAUTHORS:20120625-071919848Search for gravitational waves from intermediate mass binary black holes
https://resolver.caltech.edu/CaltechAUTHORS:20120622-112704948
Year: 2012
DOI: 10.1103/PhysRevD.85.102004
We present the results of a weakly modeled burst search for gravitational waves from mergers of nonspinning intermediate mass black holes in the total mass range 100–450 M_⊙ and with the component mass ratios between 1:1 and 4:1. The search was conducted on data collected by the LIGO and Virgo detectors between November of 2005 and October of 2007. No plausible signals were observed by the search which constrains the astrophysical rates of the intermediate mass black holes mergers as a function of the component masses. In the most efficiently detected bin centered on 88+88 M_⊙, for nonspinning sources, the rate density upper limit is 0.13 per Mpc^3 per Myr at the 90% confidence level.https://resolver.caltech.edu/CaltechAUTHORS:20120622-112704948Upper limits on a stochastic gravitational-wave background using LIGO and Virgo interferometers at 600–1000 Hz
https://resolver.caltech.edu/CaltechAUTHORS:20120703-113700056
Year: 2012
DOI: 10.1103/PhysRevD.85.122001
A stochastic background of gravitational waves is expected to arise from a superposition of many incoherent sources of gravitational waves, of either cosmological or astrophysical origin. This background is a target for the current generation of ground-based detectors. In this article we present the first joint search for a stochastic background using data from the LIGO and Virgo interferometers. In a frequency band of 600–1000 Hz, we obtained a 95% upper limit on the amplitude of Ω_(GW)(f)=Ω_3(f/900 Hz)^3, of Ω_3<0.32, assuming a value of the Hubble parameter of h_(100)=0.71. These new limits are a factor of seven better than the previous best in this frequency band.https://resolver.caltech.edu/CaltechAUTHORS:20120703-113700056The Einstein Toolkit: a community computational infrastructure for relativistic astrophysics
https://resolver.caltech.edu/CaltechAUTHORS:20120620-103906932
Year: 2012
DOI: 10.1088/0264-9381/29/11/115001
We describe the Einstein Toolkit, a community-driven, freely accessible computational infrastructure intended for use in numerical relativity, relativistic astrophysics, and other applications. The toolkit, developed by a collaboration involving researchers from multiple institutions around the world, combines a core set of components needed to simulate astrophysical objects such as black holes, compact objects, and collapsing stars, as well as a full suite of analysis tools. The Einstein Toolkit is currently based on the Cactus framework for high-performance computing and the Carpet adaptive mesh refinement driver. It implements spacetime evolution via the BSSN evolution system and general relativistic hydrodynamics in a finite-volume discretization. The toolkit is under continuous development and contains many new code components that have been publicly released for the first time and are described in this paper. We discuss the motivation behind the release of the toolkit, the philosophy underlying its development, and the goals of the project. A summary of the implemented numerical techniques is included, as are results of numerical test covering a variety of sample astrophysical problems.https://resolver.caltech.edu/CaltechAUTHORS:20120620-103906932All-sky search for gravitational-wave bursts in the second joint LIGO-Virgo run
https://resolver.caltech.edu/CaltechAUTHORS:20120723-083327977
Year: 2012
DOI: 10.1103/PhysRevD.85.122007
We present results from a search for gravitational-wave bursts in the data collected by the LIGO and Virgo detectors between July 7, 2009 and October 20, 2010: data are analyzed when at least two of the three LIGO-Virgo detectors are in coincident operation, with a total observation time of 207 days. The analysis searches for transients of duration ≲1 s over the frequency band 64–5000 Hz, without other assumptions on the signal waveform, polarization, direction or occurrence time. All identified events are consistent with the expected accidental background. We set frequentist upper limits on the rate of gravitational-wave bursts by combining this search with the previous LIGO-Virgo search on the data collected between November 2005 and October 2007. The upper limit on the rate of strong gravitational-wave bursts at the Earth is 1.3 events per year at 90% confidence. We also present upper limits on source rate density per year and Mpc^3 for sample populations of standard-candle sources. As in the previous joint run, typical sensitivities of the search in terms of the root-sum-squared strain amplitude for these waveforms lie in the range ∼5×10^(-22) Hz^(-1/2) to ∼1×10^(-20) Hz^(-1/2). The combination of the two joint runs entails the most sensitive all-sky search for generic gravitational-wave bursts and synthesizes the results achieved by the initial generation of interferometric detectors.https://resolver.caltech.edu/CaltechAUTHORS:20120723-083327977Scientific objectives of Einstein Telescope
https://resolver.caltech.edu/CaltechAUTHORS:20120803-105338433
Year: 2012
DOI: 10.1088/0264-9381/29/12/124013
The advanced interferometer network will herald a new era in observational astronomy. There is a very strong science case to go beyond the advanced detector network and build detectors that operate in a frequency range from 1 Hz to 10 kHz, with sensitivity a factor 10 better in amplitude. Such detectors will be able to probe a range of topics in nuclear physics, astronomy, cosmology and fundamental physics, providing insights into many unsolved problems in these areas.https://resolver.caltech.edu/CaltechAUTHORS:20120803-105338433Correlated Gravitational Wave and Neutrino Signals from General-Relativistic Rapidly Rotating Iron Core Collapse
https://resolver.caltech.edu/CaltechAUTHORS:20120416-080639130
Year: 2012
DOI: 10.1103/PhysRevD.86.024026
We present results from a new set of 3D general-relativistic hydrodynamic simulations of rotating iron core collapse. We assume octant symmetry and focus on axisymmetric collapse, bounce, the early postbounce evolution, and the associated gravitational wave (GW) and neutrino signals. We employ a finite-temperature nuclear equation of state, parameterized electron capture in the collapse phase, and a multi-species neutrino leakage scheme after bounce. The latter captures the important effects of deleptonization, neutrino cooling and heating and enables approximate predictions for the neutrino luminosities in the early evolution after core bounce. We consider 12_⊙ and 40_⊙ presupernova models and systematically study the effects of (i) rotation, (ii) progenitor structure, and (iii) postbounce neutrino leakage on dynamics, GW, and, neutrino signals. We demonstrate, that the GW signal of rapidly rotating core collapse is practically independent of progenitor mass and precollapse structure. Moreover, we show that the effects of neutrino leakage on the GW signal are strong only in nonrotating or slowly rotating models in which GW emission is not dominated by inner core dynamics. In rapidly rotating cores, core bounce of the centrifugally-deformed inner core excites the fundamental quadrupole pulsation mode of the nascent protoneutron star. The ensuing global oscillations (f ~700-800 Hz) lead to pronounced oscillations in the GW signal and correlated strong variations in the rising luminosities of antineutrino and heavy-lepton neutrinos. We find these features in cores that collapse to protoneutron stars with spin periods ≾ 2.5 ms and rotational energies sufficient to drive hyper-energetic core-collapse supernova explosions. Hence, joint GW + neutrino observations of a core collapse event could deliver strong evidence for or against rapid core rotation.
Our estimates suggest that the GW signal should be detectable throughout the Milky
Way by advanced laser-interferometer GW observatories, but a water-Cherenkov neutrino detector
would have to be of near-megaton size to observe the variations in the early neutrino luminosities
from a core collapse event at 1 kpc.https://resolver.caltech.edu/CaltechAUTHORS:20120416-080639130The Arduous Journey to Black Hole Formation in Potential Gamma-Ray Burst Progenitors
https://resolver.caltech.edu/CaltechAUTHORS:20120807-092710061
Year: 2012
DOI: 10.1088/0004-637X/754/1/76
We present a quantitative study on the properties at death of fast-rotating massive stars evolved at low-metallicity—objects that are proposed as likely progenitors of long-duration γ-ray bursts (LGRBs). We perform one-dimensional+rotation stellar-collapse simulations on the progenitor models of Woosley and Heger, and critically assess their potential for the formation of a black hole and a Keplerian disk (namely, a collapsar) or a proto-magnetar. We note that theoretical uncertainties in the treatment of magnetic fields and the approximate handling of rotation compromise the accuracy of stellar-evolution models. We find that only the fastest rotating progenitors achieve sufficient compactness for black hole formation while the bulk of models possess a core density structure typical of garden-variety core-collapse supernova (SN) progenitors evolved without rotation and at solar metallicity. Of the models that do have sufficient compactness for black hole formation, most of them also retain a large amount of angular momentum in the core, making them prone to a magneto-rotational explosion, therefore preferentially leaving behind a proto-magnetar. A large progenitor angular-momentum budget is often the sole criterion invoked in the community today to assess the suitability for producing a collapsar. This simplification ignores equally important considerations such as the core compactness, which conditions black hole formation, the core angular momentum, which may foster a magneto-rotational explosion preventing black hole formation, or the metallicity and the residual envelope mass which must be compatible with inferences from observed LGRB/SNe. Our study suggests that black hole formation is non-trivial, that there is room for accommodating both collapsars and proto-magnetars as LGRB progenitors, although proto-magnetars seem much more easily produced by current stellar-evolutionary models.https://resolver.caltech.edu/CaltechAUTHORS:20120807-092710061The characterization of Virgo data and its impact on gravitational-wave searches
https://resolver.caltech.edu/CaltechAUTHORS:20120816-103314301
Year: 2012
DOI: 10.1088/0264-9381/29/15/155002
Between 2007 and 2010 Virgo collected data in coincidence with the LIGO and GEO gravitational-wave (GW) detectors. These data have been searched for GWs emitted by cataclysmic phenomena in the universe, by non-axisymmetric rotating neutron stars or from a stochastic background in the frequency band of the detectors. The sensitivity of GW searches is limited by noise produced by the detector or its environment. It is therefore crucial to characterize the various noise sources in a GW detector. This paper reviews the Virgo detector noise sources, noise propagation, and conversion mechanisms which were identified in the three first Virgo observing runs. In many cases, these investigations allowed us to mitigate noise sources in the detector, or to selectively flag noise events and discard them from the data. We present examples from the joint LIGO-GEO-Virgo GW searches to show how well noise transients and narrow spectral lines have been identified and excluded from the Virgo data. We also discuss how detector characterization can improve the astrophysical reach of GW searches.https://resolver.caltech.edu/CaltechAUTHORS:20120816-103314301Implications for the Origin of GRB 051103 from LIGO Observations
https://resolver.caltech.edu/CaltechAUTHORS:20120829-154041074
Year: 2012
DOI: 10.1088/0004-637X/755/1/2
We present the results of a LIGO search for gravitational waves (GWs) associated with GRB 051103, a short-duration hard-spectrum gamma-ray burst (GRB) whose electromagnetically determined sky position is coincident with the spiral galaxy M81, which is 3.6 Mpc from Earth. Possible progenitors for short-hard GRBs include compact object mergers and soft gamma repeater (SGR) giant flares. A merger progenitor would produce a characteristic GW signal that should be detectable at a distance of M81, while GW emission from an SGR is not expected to be detectable at that distance. We found no evidence of a GW signal associated with GRB 051103. Assuming weakly beamed γ-ray emission with a jet semi-angle of 30°, we exclude a binary neutron star merger in M81 as the progenitor with a confidence of 98%. Neutron star-black hole mergers are excluded with >99% confidence. If the event occurred in M81, then our findings support the hypothesis that GRB 051103 was due to an SGR giant flare, making it one of the most distant extragalactic magnetars observed to date.https://resolver.caltech.edu/CaltechAUTHORS:20120829-154041074Inferring core-collapse supernova physics with gravitational waves
https://resolver.caltech.edu/CaltechAUTHORS:20121001-141822842
Year: 2012
DOI: 10.1103/PhysRevD.86.044023
Stellar collapse and the subsequent development of a core-collapse supernova explosion emit bursts of gravitational waves (GWs) that might be detected by the advanced generation of laser interferometer gravitational-wave observatories such as Advanced LIGO, Advanced Virgo, and LCGT. GW bursts from core-collapse supernovae encode information on the intricate multidimensional dynamics at work at the core of a dying massive star and may provide direct evidence for the yet uncertain mechanism driving supernovae in massive stars. Recent multidimensional simulations of core-collapse supernovae exploding via the neutrino, magnetorotational, and acoustic explosion mechanisms have predicted GW signals which have distinct structure in both the time and frequency domains. Motivated by this, we describe a promising method for determining the most likely explosion mechanism underlying a hypothetical GW signal, based on principal component analysis and Bayesian model selection. Using simulated Advanced LIGO noise and assuming a single detector and linear waveform polarization for simplicity, we demonstrate that our method can distinguish magnetorotational explosions throughout the Milky Way (D≲10 kpc) and explosions driven by the neutrino and acoustic mechanisms to D≲2 kpc. Furthermore, we show that we can differentiate between models for rotating accretion-induced collapse of massive white dwarfs and models of rotating iron core collapse with high reliability out to several kpc.https://resolver.caltech.edu/CaltechAUTHORS:20121001-141822842Search for Gravitational Waves Associated with Gamma-Ray Bursts during LIGO Science Run 6 and Virgo Science Runs 2 and 3
https://resolver.caltech.edu/CaltechAUTHORS:20121213-075343117
Year: 2012
DOI: 10.1088/0004-637X/760/1/12
We present the results of a search for gravitational waves associated with 154 gamma-ray bursts (GRBs) that were detected by satellite-based gamma-ray experiments in 2009-2010, during the sixth LIGO science run and the second and third Virgo science runs. We perform two distinct searches: a modeled search for coalescences of either two neutron stars or a neutron star and black hole, and a search for generic, unmodeled gravitational-wave bursts. We find no evidence for gravitational-wave counterparts, either with any individual GRB in this sample or with the population as a whole. For all GRBs we place lower bounds on the distance to the progenitor, under the optimistic assumption of a gravitational-wave emission energy of 10^(–2) M_☉ c^2 at 150 Hz, with a median limit of 17 Mpc. For short-hard GRBs we place exclusion distances on binary neutron star and neutron-star-black-hole progenitors, using astrophysically motivated priors on the source parameters, with median values of 16 Mpc and 28 Mpc, respectively. These distance limits, while significantly larger than for a search that is not aided by GRB satellite observations, are not large enough to expect a coincidence with a GRB. However, projecting these exclusions to the sensitivities of Advanced LIGO and Virgo, which should begin operation in 2015, we find that the detection of gravitational waves associated with GRBs will become quite possible.https://resolver.caltech.edu/CaltechAUTHORS:20121213-075343117Charged-current neutrino interactions in core-collapse supernovae in a virial expansion
https://resolver.caltech.edu/CaltechAUTHORS:20130117-102255248
Year: 2012
DOI: 10.1103/PhysRevC.86.065806
Core-collapse supernovae may depend sensitively on charged-current neutrino interactions in warm, low-density, neutron-rich matter. A proton in neutron-rich matter is more tightly bound than is a neutron. This energy shift ΔU increases the electron energy in ν_e + n → p + e, increasing the available phase space and absorption cross section. Likewise ΔU decreases the positron energy in ν̅_ e + p → n + e+, decreasing the phase space and cross section. We have calculated ΔU using a model-independent virial expansion and we find that ΔU is much larger, at low densities, than the predictions of many mean-field models. Therefore ΔU could have a significant impact on charged-current neutrino interactions in supernovae. Preliminary simulations of the accretion phase of core-collapse supernovae find that ΔU increases ν̅ _e energies and decreases the ν_e luminosity.https://resolver.caltech.edu/CaltechAUTHORS:20130117-102255248Swift Follow-up Observations of Candidate Gravitational-wave Transient Events
https://resolver.caltech.edu/CaltechAUTHORS:20130114-104018011
Year: 2012
DOI: 10.1088/0067-0049/203/2/28
We present the first multi-wavelength follow-up observations of two candidate gravitational-wave (GW) transient events recorded by LIGO and Virgo in their 2009-2010 science run. The events were selected with low latency by the network of GW detectors (within less than 10 minutes) and their candidate sky locations were observed by the Swift observatory (within 12 hr). Image transient detection was used to analyze the collected electromagnetic data, which were found to be consistent with background. Off-line analysis of the GW data alone has also established that the selected GW events show no evidence of an astrophysical origin; one of them is consistent with background and the other one was a test, part of a "blind injection challenge." With this work we demonstrate the feasibility of rapid follow-ups of GW transients and establish the sensitivity improvement joint electromagnetic and GW observations could bring. This is a first step toward an electromagnetic follow-up program in the regime of routine detections with the advanced GW instruments expected within this decade. In that regime, multi-wavelength observations will play a significant role in completing the astrophysical identification of GW sources. We present the methods and results from this first combined analysis and discuss its implications in terms of sensitivity for the present and future instruments.https://resolver.caltech.edu/CaltechAUTHORS:20130114-104018011The Progenitor Dependence of the Preexplosion Neutrino Emission in Core-Collapse Supernovae
https://resolver.caltech.edu/CaltechAUTHORS:20120723-090845964
Year: 2013
DOI: 10.1088/0004-637X/762/2/126
We perform spherically symmetric general-relativistic simulations of core collapse and the postbounce pre-explosion phase in 32 presupernova stellar models of solar metallicity with zero-age main-sequence masses of 12-120 M_☉. Using energy-dependent three-species neutrino transport in the two-moment approximation with an analytic closure, we show that the emitted neutrino luminosities and spectra follow very systematic trends that are correlated with the compactness (~M/R) of the progenitor star's inner regions via the accretion rate in the pre-explosion phase. We find that these qualitative trends depend only weakly on the nuclear equation of state (EOS), but quantitative observational statements will require independent constraints on the EOS and the rotation rate of the core as well as a more complete understanding of neutrino oscillations. We investigate the simulated response of water Cherenkov detectors to the electron antineutrino fluxes from our models and find that the large statistics of a galactic core collapse event may allow robust conclusions on the inner structure of the progenitor star.https://resolver.caltech.edu/CaltechAUTHORS:20120723-090845964Search for gravitational waves from binary black hole inspiral, merger, and ringdown in LIGO-Virgo data from 2009–2010
https://resolver.caltech.edu/CaltechAUTHORS:20130221-081251760
Year: 2013
DOI: 10.1103/PhysRevD.87.022002
We report a search for gravitational waves from the inspiral, merger and ringdown of binary black holes (BBH) with total mass between 25 and 100 solar masses, in data taken at the LIGO and Virgo observatories between July 7, 2009 and October 20, 2010. The maximum sensitive distance of the detectors over this period for a (20,20)M_⊙ coalescence was 300 Mpc. No gravitational wave signals were found. We thus report upper limits on the astrophysical coalescence rates of BBH as a function of the component masses for nonspinning components, and also evaluate the dependence of the search sensitivity on component spins aligned with the orbital angular momentum. We find an upper limit at 90% confidence on the coalescence rate of BBH with nonspinning components of mass between 19 and 28M_⊙ of 3.3×10^(-7) mergers Mpc^(-3) yr^(-1).https://resolver.caltech.edu/CaltechAUTHORS:20130221-081251760Core-Collapse Supernovae, Neutrinos, and Gravitational Waves
https://resolver.caltech.edu/CaltechAUTHORS:20130208-113632514
Year: 2013
DOI: 10.1016/j.nuclphysbps.2013.04.036
Core-collapse supernovae are among the most energetic cosmic cataclysms. They are prodigious emitters of neutrinos
and quite likely strong galactic sources of gravitational waves. Observation of both neutrinos and gravitational
waves from the next galactic or near extragalactic core-collapse supernova will yield a wealth of information on the
explosion mechanism, but also on the structure and angular momentum of the progenitor star, and on aspects of
fundamental physics such as the equation of state of nuclear matter at high densities and low entropies. In this contribution
to the proceedings of the Neutrino 2012 conference, we summarize recent progress made in the theoretical
understanding and modeling of core-collapse supernovae. In this, our emphasis is on multi-dimensional processes
involved in the explosion mechanism such as neutrino-driven convection and the standing accretion shock instability.
As an example of how supernova neutrinos can be used to probe fundamental physics, we discuss how the rise time
of the electron antineutrino flux observed in detectors can be used to probe the neutrino mass hierarchy. Finally, we
lay out aspects of the neutrino and gravitational-wave signature of core-collapse supernovae and discuss the power of
combined analysis of neutrino and gravitational wave data from the next galactic core-collapse supernova.https://resolver.caltech.edu/CaltechAUTHORS:20130208-113632514Einstein@Home all-sky search for periodic gravitational waves in LIGO S5 data
https://resolver.caltech.edu/CaltechAUTHORS:20130327-074611717
Year: 2013
DOI: 10.1103/PhysRevD.87.042001
This paper presents results of an all-sky search for periodic gravitational waves in the frequency range [50,1 190] Hz and with frequency derivative range of ∼[-20,1.1]×10^(-10) Hz s^(-1) for the fifth LIGO science run (S5). The search uses a noncoherent Hough-transform method to combine the information from coherent searches on time scales of about one day. Because these searches are very computationally intensive, they have been carried out with the Einstein@Home volunteer distributed computing project. Postprocessing identifies eight candidate signals; deeper follow-up studies rule them out. Hence, since no gravitational wave signals have been found, we report upper limits on the intrinsic gravitational wave strain amplitude h_0. For example, in the 0.5 Hz-wide band at 152.5 Hz, we can exclude the presence of signals with h_0 greater than 7.6×10^(-25) at a 90% confidence level. This search is about a factor 3 more sensitive than the previous Einstein@Home search of early S5 LIGO data.https://resolver.caltech.edu/CaltechAUTHORS:20130327-074611717Three-dimensional general-relativistic hydrodynamic simulations of binary neutron star coalescence and stellar collapse with multipatch grids
https://resolver.caltech.edu/CaltechAUTHORS:20130208-133528473
Year: 2013
DOI: 10.1103/PhysRevD.87.064023
We present a new three-dimensional, general-relativistic hydrodynamic evolution scheme coupled to dynamical spacetime evolutions which is capable of efficiently simulating stellar collapse, isolated neutron stars, black hole formation, and binary neutron star coalescence. We make use of a set of adapted curvilinear grids (multipatches) coupled with flux-conservative, cell-centered adaptive mesh refinement. This allows us to significantly enlarge our computational domains while still maintaining high resolution in the gravitational wave extraction zone, the exterior layers of a star, or the region of mass ejection in merging neutron stars. The fluid is evolved with a high-resolution, shock-capturing finite volume scheme, while the spacetime geometry is evolved using fourth-order finite differences. We employ a multirate Runge-Kutta time-integration scheme for efficiency, evolving the fluid with second-order integration and the spacetime geometry with fourth-order integration. We validate our code by a number of benchmark problems: a rotating stellar collapse model, an excited neutron star, neutron star collapse to a black hole, and binary neutron star coalescence. The test problems, especially the latter, greatly benefit from higher resolution in the gravitational wave extraction zone, causally disconnected outer boundaries, and application of Cauchy-characteristic gravitational wave extraction. We show that we are able to extract convergent gravitational wave modes up to (ℓ,m)=(6,6). This study paves the way for more realistic and detailed studies of compact objects and stellar collapse in full three dimensions and in large computational domains. The multipatch infrastructure and the improvements to mesh refinement and hydrodynamics codes discussed in this paper will be made available as part of the open-source Einstein Toolkit.https://resolver.caltech.edu/CaltechAUTHORS:20130208-133528473Black-hole–neutron-star mergers at realistic mass ratios: Equation of state and spin orientation effects
https://resolver.caltech.edu/CaltechAUTHORS:20130506-154544306
Year: 2013
DOI: 10.1103/PhysRevD.87.084006
Black-hole–neutron-star mergers resulting in the disruption of the neutron star and the formation of an accretion disk and/or the ejection of unbound material are prime candidates for the joint detection of gravitational-wave and electromagnetic signals when the next generation of gravitational-wave detectors comes online. However, the disruption of the neutron star and the properties of the postmerger remnant are very sensitive to the parameters of the binary (mass ratio, black-hole spin, neutron star radius). In this paper, we study the impact of the radius of the neutron star and the alignment of the black-hole spin on black-hole–neutron-star mergers within the range of mass ratio currently deemed most likely for field binaries (M_BH∼7M_NS) and for black-hole spins large enough for the neutron star to disrupt (J_BH/M^(2)_(BH)=0.9). We find that (i) In this regime, the merger is particularly sensitive to the radius of the neutron star, with remnant masses varying from 0.3M_NS to 0.1M_NS for changes of only 2 km in the NS radius; (ii) 0.01M_(⊙)–0.05M_(⊙) of unbound material can be ejected with kinetic energy ≳10^(51) ergs, a significant increase compared to low mass ratio, low spin binaries. This ejecta could power detectable postmerger optical and radio afterglows. (iii) Only a small fraction of the Advanced LIGO events in this parameter range have gravitational-wave signals which could offer constraints on the equation of state of the neutron star (at best ∼3% of the events for a single detector at design sensitivity). (iv) A misaligned black-hole spin works against disk formation, with less neutron-star material remaining outside of the black hole after merger, and a larger fraction of that material remaining in the tidal tail instead of the forming accretion disk. (v) Large kicks v_kick≳300 km/s can be given to the final black hole as a result of a precessing black-hole–neutron-star merger, when the disruption of the neutron star occurs just outside or within the innermost stable spherical orbit.https://resolver.caltech.edu/CaltechAUTHORS:20130506-154544306The runaway instability in general relativistic accretion discs
https://resolver.caltech.edu/CaltechAUTHORS:20130212-090441321
Year: 2013
DOI: 10.1093/mnras/stt166
When an accretion disc falls prey to the runaway instability, a large portion of its mass is devoured by the black hole within a few dynamical times. Despite decades of effort, it is still unclear under what conditions such an instability can occur. The technically most advanced relativistic simulations to date were unable to find a clear sign for the onset of the instability. In this work, we present three-dimensional relativistic hydrodynamics simulations of accretion discs around black holes in dynamical space–time. We focus on the configurations that are expected to be particularly prone to the development of this instability. We demonstrate, for the first time, that the fully self-consistent general relativistic evolution does indeed produce a runaway instability.https://resolver.caltech.edu/CaltechAUTHORS:20130212-090441321General-relativistic Simulations of Three-dimensional Core-collapse Supernovae
https://resolver.caltech.edu/CaltechAUTHORS:20130604-143255579
Year: 2013
DOI: 10.1088/0004-637X/768/2/115
We study the three-dimensional (3D) hydrodynamics of the post-core-bounce phase of the collapse of a 27 M_☉ star and pay special attention to the development of the standing accretion shock instability (SASI) and neutrino-driven convection. To this end, we perform 3D general-relativistic simulations with a three-species neutrino leakage scheme. The leakage scheme captures the essential aspects of neutrino cooling, heating, and lepton number exchange as predicted by radiation-hydrodynamics simulations. The 27 M_☉ progenitor was studied in 2D by Müller et al., who observed strong growth of the SASI while neutrino-driven convection was suppressed. In our 3D simulations, neutrino-driven convection grows from numerical perturbations imposed by our Cartesian grid. It becomes the dominant instability and leads to large-scale non-oscillatory deformations of the shock front. These will result in strongly aspherical explosions without the need for large-scale SASI shock oscillations. Low-ℓ-mode SASI oscillations are present in our models, but saturate at small amplitudes that decrease with increasing neutrino heating and vigor of convection. Our results, in agreement with simpler 3D Newtonian simulations, suggest that once neutrino-driven convection is started, it is likely to become the dominant instability in 3D. Whether it is the primary instability after bounce will ultimately depend on the physical seed perturbations present in the cores of massive stars. The gravitational wave signal, which we extract and analyze for the first time from 3D general-relativistic models, will serve as an observational probe of the postbounce dynamics and, in combination with neutrinos, may allow us to determine the primary hydrodynamic instability.https://resolver.caltech.edu/CaltechAUTHORS:20130604-143255579A first search for coincident gravitational waves and high energy neutrinos using LIGO, Virgo and ANTARES data from 2007
https://resolver.caltech.edu/CaltechAUTHORS:20131111-140413004
Year: 2013
DOI: 10.1088/1475-7516/2013/06/008
We present the results of the first search for gravitational wave bursts associated with high energy neutrinos. Together, these messengers could reveal new, hidden sources that are not observed by conventional photon astronomy, particularly at high energy. Our search uses neutrinos detected by the underwater neutrino telescope ANTARES in its 5 line configuration during the period January - September 2007, which coincided with the fifth and first science runs of LIGO and Virgo, respectively. The LIGO-Virgo data were analysed for candidate gravitational-wave signals coincident in time and direction with the neutrino events. No significant coincident events were observed. We place limits on the density of joint high energy neutrino - gravitational wave emission events in the local universe, and compare them with densities of merger and core-collapse events.https://resolver.caltech.edu/CaltechAUTHORS:20131111-140413004A New Spherical Harmonics Scheme for Multi-Dimensional Radiation Transport I: Static Matter Configurations
https://resolver.caltech.edu/CaltechAUTHORS:20130212-111657555
Year: 2013
DOI: 10.1016/j.jcp.2013.01.048
Recent work by McClarren and Hauck (2010) [31] suggests that the filtered spherical
harmonics method represents an efficient, robust, and accurate method for radiation
transport, at least in the two-dimensional (2D) case. We extend their work to the threedimensional
(3D) case and find that all of the advantages of the filtering approach identified
in 2D are present also in the 3D case. We reformulate the filter operation in a way that
is independent of the timestep and of the spatial discretization. We also explore different
second- and fourth-order filters and find that the second-order ones yield significantly
better results. Overall, our findings suggest that the filtered spherical harmonics approach
represents a very promising method for 3D radiation transport calculations.https://resolver.caltech.edu/CaltechAUTHORS:20130212-111657555Enhanced sensitivity of the LIGO gravitational wave detector by using squeezed states of light
https://resolver.caltech.edu/CaltechAUTHORS:20140124-105938310
Year: 2013
DOI: 10.1038/NPHOTON.2013.177
Nearly a century after Einstein first predicted the existence of gravitational waves, a global network of Earth-based gravitational wave observatories is seeking to directly detect this faint radiation using precision laser interferometry. Photon shot noise, due to the quantum nature of light, imposes a fundamental limit on the attometre-level sensitivity of the kilometre-scale Michelson interferometers deployed for this task. Here, we inject squeezed states to improve the performance of one of the detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) beyond the quantum noise limit, most notably in the frequency region down to 150 Hz, critically important for several astrophysical sources, with no deterioration of performance observed at any frequency. With the injection of squeezed states, this LIGO detector demonstrated the best broadband sensitivity to gravitational waves ever achieved, with important implications for observing the gravitational-wave Universe with unprecedented sensitivity.https://resolver.caltech.edu/CaltechAUTHORS:20140124-105938310Parameter estimation for compact binary coalescence signals with the first generation gravitational-wave detector network
https://resolver.caltech.edu/CaltechAUTHORS:20131021-100815172
Year: 2013
DOI: 10.1103/PhysRevD.88.062001
Compact binary systems with neutron stars or black holes are one of the most promising sources for ground-based gravitational-wave detectors. Gravitational radiation encodes rich information about source physics; thus parameter estimation and model selection are crucial analysis steps for any detection candidate events. Detailed models of the anticipated waveforms enable inference on several parameters, such as component masses, spins, sky location and distance, that are essential for new astrophysical studies of these sources. However, accurate measurements of these parameters and discrimination of models describing the underlying physics are complicated by artifacts in the data, uncertainties in the waveform models and in the calibration of the detectors. Here we report such measurements on a selection of simulated signals added either in hardware or software to the data collected by the two LIGO instruments and the Virgo detector during their most recent joint science run, including a "blind injection" where the signal was not initially revealed to the collaboration. We exemplify the ability to extract information about the source physics on signals that cover the neutron-star and black-hole binary parameter space over the component mass range 1 M_⊙–25 M_⊙ and the full range of spin parameters. The cases reported in this study provide a snapshot of the status of parameter estimation in preparation for the operation of advanced detectors.https://resolver.caltech.edu/CaltechAUTHORS:20131021-100815172Black Hole-Neutron Star Mergers with a Hot Nuclear Equation of State: Outflow and Neutrino-cooled Disk for a Low-mass, High-spin Case
https://resolver.caltech.edu/CaltechAUTHORS:20131101-114026087
Year: 2013
DOI: 10.1088/0004-637X/776/1/47
Neutrino emission significantly affects the evolution of the accretion tori formed in black hole-neutron star mergers. It removes energy from the disk, alters its composition, and provides a potential power source for a gamma-ray burst. To study these effects, simulations in general relativity with a hot microphysical equation of state (EOS) and neutrino feedback are needed. We present the first such simulation, using a neutrino leakage scheme for cooling to capture the most essential effects and considering a moderate mass (1.4 M_☉ neutron star, 5.6 M_☉ black hole), high-spin (black hole J/M^2 = 0.9) system with the K_0 = 220 MeV Lattimer-Swesty EOS. We find that about 0.08 M_☉ of nuclear matter is ejected from the system, while another 0.3 M_☉ forms a hot, compact accretion disk. The primary effects of the escaping neutrinos are (1) to make the disk much denser and more compact, (2) to cause the average electron fraction Ye of the disk to rise to about 0.2 and then gradually decrease again, and (3) to gradually cool the disk. The disk is initially hot (T ~ 6 MeV) and luminous in neutrinos (L_ν ~ 10^54 erg s^–1), but the neutrino luminosity decreases by an order of magnitude over 50 ms of post-merger evolution.https://resolver.caltech.edu/CaltechAUTHORS:20131101-114026087Formation and Coalescence of Cosmological Supermassive-Black-Hole Binaries in Supermassive-Star Collapse
https://resolver.caltech.edu/CaltechAUTHORS:20131106-100213472
Year: 2013
DOI: 10.1103/PhysRevLett.111.151101
We study the collapse of rapidly rotating supermassive stars that may have formed in the early Universe. By self-consistently simulating the dynamics from the onset of collapse using three-dimensional general-relativistic hydrodynamics with fully dynamical spacetime evolution, we show that seed perturbations in the progenitor can lead to the formation of a system of two high-spin supermassive black holes, which inspiral and merge under the emission of powerful gravitational radiation that could be observed at redshifts z≳10 with the DECIGO or Big Bang Observer gravitational-wave observatories, assuming supermassive stars in the mass range 10^4–10^6M⊙. The remnant is rapidly spinning with dimensionless spin a*=0.9. The surrounding accretion disk contains ∼10% of the initial mass.https://resolver.caltech.edu/CaltechAUTHORS:20131106-100213472Revival of the Stalled Core-collapse Supernova Shock Triggered by Precollapse Asphericity in the Progenitor Star
https://resolver.caltech.edu/CaltechAUTHORS:20131223-075453787
Year: 2013
DOI: 10.1088/2041-8205/778/1/L7
Multi-dimensional simulations of advanced nuclear burning stages of massive stars suggest that the Si/O layers of presupernova stars harbor large deviations from the spherical symmetry typically assumed for presupernova stellar structure. We carry out three-dimensional core-collapse supernova simulations with and without aspherical velocity perturbations to assess their potential impact on the supernova hydrodynamics in the stalled-shock phase. Our results show that realistic perturbations can qualitatively alter the postbounce evolution, triggering an explosion in a model that fails to explode without them. This finding underlines the need for a multi-dimensional treatment of the presupernova stage of stellar evolution.https://resolver.caltech.edu/CaltechAUTHORS:20131223-075453787Search for long-lived gravitational-wave transients coincident with long gamma-ray bursts
https://resolver.caltech.edu/CaltechAUTHORS:20140130-095945569
Year: 2013
DOI: 10.1103/PhysRevD.88.122004
Long gamma-ray bursts (GRBs) have been linked to extreme core-collapse supernovae from massivestars. Gravitational waves (GW) offer a probe of the physics behind long GRBs. We investigate models of long-lived (~10–1000 s) GW emission associated with the accretion disk of a collapsed star or with its
protoneutron star remnant. Using data from LIGO's fifth science run, and GRB triggers from the Swift experiment, we perform a search for unmodeled long-lived GW transients. Finding no evidence of GW emission, we place 90% confidence-level upper limits on the GW fluence at Earth from long GRBs for
three waveforms inspired by a model of GWs from accretion disk instabilities. These limits range from F < 3:5 ergs cm^(-2) to F < 1200 ergs cm^(-2), depending on the GRB and on the model, allowing us to probe
optimistic scenarios of GW production out to distances as far as ≈ 33 Mpc. Advanced detectors are expected to achieve strain sensitivities 10x better than initial LIGO, potentially allowing us to probe the engines of the nearest long GRBs.https://resolver.caltech.edu/CaltechAUTHORS:20140130-095945569GRHydro: a new open-source general-relativistic magnetohydrodynamics code for the Einstein toolkit
https://resolver.caltech.edu/CaltechAUTHORS:20131224-073746921
Year: 2014
DOI: 10.1088/0264-9381/31/1/015005
We present the new general-relativistic magnetohydrodynamics (GRMHD) capabilities of the Einstein toolkit, an open-source community-driven numerical relativity and computational relativistic astrophysics code. The GRMHD extension of the toolkit builds upon previous releases and implements the evolution of relativistic magnetized fluids in the ideal MHD limit in fully dynamical spacetimes using the same shock-capturing techniques previously applied to hydrodynamical evolution. In order to maintain the divergence-free character of the magnetic field, the code implements both constrained transport and hyperbolic divergence cleaning schemes. We present test results for a number of MHD tests in Minkowski and curved spacetimes. Minkowski tests include aligned and oblique planar shocks, cylindrical explosions, magnetic rotors, Alfvén waves and advected loops, as well as a set of tests designed to study the response of the divergence cleaning scheme to numerically generated monopoles. We study the code's performance in curved spacetimes with spherical accretion onto a black hole on a fixed background spacetime and in fully dynamical spacetimes by evolutions of a magnetized polytropic neutron star and of the collapse of a magnetized stellar core. Our results agree well with exact solutions where these are available and we demonstrate convergence. All code and input files used to generate the results are available on http://einsteintoolkit.org. This makes our work fully reproducible and provides new users with an introduction to applications of the code.https://resolver.caltech.edu/CaltechAUTHORS:20131224-073746921Magnetorotational Core-collapse Supernovae in Three Dimensions
https://resolver.caltech.edu/CaltechAUTHORS:20140424-194106351
Year: 2014
DOI: 10.1088/2041-8205/785/2/L29
We present results of new three-dimensional (3D) general-relativistic magnetohydrodynamic simulations of rapidly rotating strongly magnetized core collapse. These simulations are the first of their kind and include a microphysical finite-temperature equation of state and a leakage scheme that captures the overall energetics and lepton number exchange due to postbounce neutrino emission. Our results show that the 3D dynamics of magnetorotational core-collapse supernovae are fundamentally different from what was anticipated on the basis of previous simulations in axisymmetry (2D). A strong bipolar jet that develops in a simulation constrained to 2D is crippled by a spiral instability and fizzles in full 3D. While multiple (magneto-)hydrodynamic instabilities may be present, our analysis suggests that the jet is disrupted by an m = 1 kink instability of the ultra-strong toroidal field near the rotation axis. Instead of an axially symmetric jet, a completely new, previously unreported flow structure develops. Highly magnetized spiral plasma funnels expelled from the core push out the shock in polar regions, creating wide secularly expanding lobes. We observe no runaway explosion by the end of the full 3D simulation 185 ms after bounce. At this time, the lobes have reached maximum radii of ~900 km.https://resolver.caltech.edu/CaltechAUTHORS:20140424-194106351Application of a Hough search for continuous gravitational waves on data from the fifth LIGO science run
https://resolver.caltech.edu/CaltechAUTHORS:20140603-092556549
Year: 2014
DOI: 10.1088/0264-9381/31/8/085014
We report on an all-sky search for periodic gravitational waves in the frequency range 50–1000 Hz with the first derivative of frequency in the range −8.9 × 10^(−10) Hz s^(−1) to zero in two years of data collected during LIGO's fifth science run. Our results employ a Hough transform technique, introducing a χ^2 test and analysis of coincidences between the signal levels in years 1 and 2 of observations that offers a significant improvement in the product of strain sensitivity with compute cycles per data sample compared to previously published searches. Since our search yields no surviving candidates, we present results taking the form of frequency dependent, 95% confidence upper limits on the strain amplitude h0. The most stringent upper limit from year 1 is 1.0 × 10^(−24) in the 158.00–158.25 Hz band. In year 2, the most stringent upper limit is 8.9 × 10^(−25) in the 146.50–146.75 Hz band. This improved detection pipeline, which is computationally efficient by at least two orders of magnitude better than our flagship Einstein@Home search, will be important for 'quick-look' searches in the Advanced LIGO and Virgo detector era.https://resolver.caltech.edu/CaltechAUTHORS:20140603-092556549Neutron star-black hole mergers with a nuclear equation of state and neutrino cooling: Dependence in the binary parameters
https://resolver.caltech.edu/CaltechAUTHORS:20140819-105645070
Year: 2014
DOI: 10.1103/PhysRevD.90.024026
We present a first exploration of the results of neutron star-black hole mergers using black hole masses in the most likely range of 7M_⊙ –10M_⊙, a neutrino leakage scheme, and a modeling of the neutron star material through a finite-temperature nuclear-theory based equation of state. In the range of black hole spins in which the neutron star is tidally disrupted (χ BH ≳0.7), we show that the merger consistently produces large amounts of cool (T≲1 MeV), unbound, neutron-rich material (M_(ej) ∼ 0.05M_⊙ –0.20M_⊙). A comparable amount of bound matter is initially divided between a hot disk (T_(max) ∼15 MeV) with typical neutrino luminosity of L_ ν ∼10^(53) erg/s, and a cooler tidal tail. After a short period of rapid protonization of the disk lasting ∼10 ms, the accretion disk cools down under the combined effects of the fall-back of cool material from the tail, continued accretion of the hottest material onto the black hole, and neutrino emission. As the temperature decreases, the disk progressively becomes more neutron rich, with dimmer neutrino emission. This cooling process should stop once the viscous heating in the disk (not included in our simulations) balances the cooling. These mergers of neutron star-black hole binaries with black hole masses of M_(BH) ∼7M_⊙ –10M_⊙, and black hole spins high enough for the neutron star to disrupt provide promising candidates for the production of short gamma-ray bursts, of bright infrared postmerger signals due to the radioactive decay of unbound material, and of large amounts of r-process nuclei.https://resolver.caltech.edu/CaltechAUTHORS:20140819-105645070The Influence of Thermal Pressure on Equilibrium Models of Hypermassive Neutron Star Merger Remnants
https://resolver.caltech.edu/CaltechAUTHORS:20140814-142315469
Year: 2014
DOI: 10.1088/0004-637X/790/1/19
The merger of two neutron stars leaves behind a rapidly spinning hypermassive object whose survival is believed to depend on the maximum mass supported by the nuclear equation of state (EOS), angular momentum redistribution by (magneto-)rotational instabilities, and spindown by gravitational waves. The high temperatures (~5-40 MeV) prevailing in the merger remnant may provide thermal pressure support that could increase its maximum mass and, thus, its life on a neutrino-cooling timescale. We investigate the role of thermal pressure support in hypermassive merger remnants by computing sequences of spherically symmetric and axisymmetric uniformly and differentially rotating equilibrium solutions to the general-relativistic stellar structure equations. Using a set of finite-temperature nuclear EOS, we find that hot maximum-mass critically spinning configurations generally do not support larger baryonic masses than their cold counterparts. However, subcritically spinning configurations with mean density of less than a few times nuclear saturation density yield a significantly thermally enhanced mass. Even without decreasing the maximum mass, cooling and other forms of energy loss can drive the remnant to an unstable state. We infer secular instability by identifying approximate energy turning points in equilibrium sequences of constant baryonic mass parameterized by maximum density. Energy loss carries the remnant along the direction of decreasing gravitational mass and higher density until instability triggers collapse. Since configurations with more thermal pressure support are less compact and thus begin their evolution at a lower maximum density, they remain stable for longer periods after merger.https://resolver.caltech.edu/CaltechAUTHORS:20140814-142315469Measuring the angular momentum distribution in core-collapse supernova progenitors with gravitational waves
https://resolver.caltech.edu/CaltechAUTHORS:20140904-123411118
Year: 2014
DOI: 10.1103/PhysRevD.90.044001
The late collapse, core bounce, and the early postbounce phase of rotating core collapse leads to a characteristic gravitational wave (GW) signal. The precise shape of the signal is governed by the interplay of gravity, rotation, nuclear equation of state (EOS), and electron capture during collapse. We explore the detailed dependence of the signal on total angular momentum and its distribution in the progenitor core by means of a large set of axisymmetric general-relativistic hydrodynamics core-collapse simulations, in which we systematically vary the initial angular momentum distribution in the core. Our simulations include a microphysical finite-temperature EOS, an approximate electron capture treatment during collapse, and a neutrino leakage scheme for the postbounce evolution. Our results show that the total angular momentum of the inner core at bounce and the inner core's ratio of rotational kinetic energy to gravitational energy T/|W| are both robust parameters characterizing the GW signal. We find that the precise distribution of angular momentum is relevant only for very rapidly rotating cores with T/|W|≳8% at bounce. We construct a numerical template bank from our baseline set of simulations, and carry out additional simulations to generate trial waveforms for injection into simulated Advanced LIGO noise at a fiducial galactic distance of 10 kpc. Using matched filtering, we show that for an optimally oriented source and Gaussian noise, Advanced LIGO could measure the total angular momentum to within ±20%, for rapidly rotating cores. For most waveforms, the nearest known degree of precollapse differential rotation is correctly inferred by both our matched filtering analysis and an alternative Bayesian model selection approach. We test our results for robustness against systematic uncertainties by injecting waveforms from simulations utilizing a different EOS and variations in the electron fraction in the inner core. The results of these tests show that these uncertainties significantly reduce the accuracy with which the total angular momentum and its precollapse distribution can be inferred from observations.https://resolver.caltech.edu/CaltechAUTHORS:20140904-123411118Magnetic effects on the low-T/|W| instability in differentially rotating neutron stars
https://resolver.caltech.edu/CaltechAUTHORS:20150107-084235303
Year: 2014
DOI: 10.1103/PhysRevD.90.104014
Dynamical instabilities in protoneutron stars may produce gravitational waves whose observation could shed light on the physics of core-collapse supernovae. When born with sufficient differential rotation, these stars are susceptible to a shear instability (the "low-T/|W| instability"), but such rotation can also amplify magnetic fields to strengths where they have a considerable impact on the dynamics of the stellar matter. Using a new magnetohydrodynamics module for the Spectral Einstein Code, we have simulated a differentially-rotating neutron star in full 3D to study the effects of magnetic fields on this instability. Though strong toroidal fields were predicted to suppress the low-T/|W| instability, we find that they do so only in a small range of field strengths. Below 4×10^(13) G, poloidal seed fields do not wind up fast enough to have an effect before the instability saturates, while above 5×10^(14) G, magnetic instabilities can actually amplify a global quadrupole mode (this threshold may be even lower in reality, as small-scale magnetic instabilities remain difficult to resolve numerically). Thus, the prospects for observing gravitational waves from such systems are not in fact diminished over most of the magnetic parameter space. Additionally, we report that the detailed development of the low-T/|W| instability, including its growth rate, depends strongly on the particular numerical methods used. The high-order methods we employ suggest that growth might be considerably slower than found in some previous simulations.https://resolver.caltech.edu/CaltechAUTHORS:20150107-084235303Multivariate Regression Analysis of Gravitational Waves from Rotating Core Collapse
https://resolver.caltech.edu/CaltechAUTHORS:20150112-142914130
Year: 2014
DOI: 10.1103/PhysRevD.90.124026
We present a new multivariate regression model for analysis and parameter
estimation of gravitational waves observed from well but not perfectly modeled
sources such as core-collapse supernovae. Our approach is based on a principal
component decomposition of simulated waveform catalogs. Instead of
reconstructing waveforms by direct linear combination of physically meaningless
principal components, we solve via least squares for the relationship that
encodes the connection between chosen physical parameters and the principal
component basis. Although our approach is linear, the waveforms' parameter
dependence may be non-linear. For the case of gravitational waves from rotating
core collapse, we show, using statistical hypothesis testing, that our method
is capable of identifying the most important physical parameters that govern
waveform morphology in the presence of simulated detector noise. We also
demonstrate our method's ability to predict waveforms from a principal
component basis given a set of physical progenitor parameters.https://resolver.caltech.edu/CaltechAUTHORS:20150112-142914130The Role of Turbulence in Neutrino-Driven Core-Collapse Supernova Explosions
https://resolver.caltech.edu/CaltechAUTHORS:20141216-132735192
Year: 2015
DOI: 10.1088/0004-637X/799/1/5
The neutrino-heated "gain layer" immediately behind the stalled shock in a
core-collapse supernova is unstable to high-Reynolds-number turbulent
convection. We carry out and analyze a new set of 19 high-resolution
three-dimensional (3D) simulations with a three-species neutrino
leakage/heating scheme and compare with spherically-symmetric (1D) and
axisymmetric (2D) simulations carried out with the same methods. We study the postbounce supernova evolution in a 15-M_⊙ progenitor star and vary the
local neutrino heating rate, the magnitude and spatial dependence of
asphericity from convective burning in the Si/O shell, and spatial resolution.
Our simulations suggest that there is a direct correlation between the strength of turbulence in the gain layer and the susceptability to explosion. 2D and 3D simulations explode at much lower neutrino heating rates than 1D simulations.
This is commonly explained by the fact that nonradial dynamics allows accreting
material to stay longer in the gain layer. We show that this explanation is
incomplete. Our results indicate that the effective turbulent ram pressure
exerted on the shock plays a crucial role by allowing multi-D models to explode
at a lower postshock thermal pressure and thus with less neutrino heating than
1D models. We connect the turbulent ram pressure with turbulent energy at large
scales and in this way explain why 2D simulations are erroneously exploding
more easily than 3D simulations.https://resolver.caltech.edu/CaltechAUTHORS:20141216-132735192The Black Hole Formation Probability
https://resolver.caltech.edu/CaltechAUTHORS:20141216-133549029
Year: 2015
DOI: 10.1088/0004-637X/799/2/190
A longstanding question in stellar evolution is which massive stars produce
black holes (BHs) rather than neutron stars (NSs) upon death. It has been
common practice to assume that a given zero-age main sequence (ZAMS) mass star (and perhaps a given metallicity) simply produces either an NS or a BH, but this fails to account for a myriad of other variables that may effect this outcome, such as spin, binarity, or even stochastic differences in the stellar structure near core collapse. We argue that instead a probabilistic description of NS versus BH formation may be better suited to account for the current uncertainties in understanding how massive stars die. We present an initial exploration of the probability that a star will make a BH as a function of its ZAMS mass, P_(BH)(M_(ZAMS)). Although we find that it is difficult to derive a unique P_(BH)(M_(ZAMS)) using current measurements of both the BH mass distribution and the degree of chemical enrichment by massive stars, we demonstrate how P_(BH)(M_(ZAMS)) changes with these various observational and theoretical uncertainties. We anticipate that future studies of Galactic BHs and theoretical studies of core collapse will refine P_(BH)(M_(ZAMS)) and argue that this framework is an important new step toward better understanding BH formation. A probabilistic description of BH formation will be useful as input for future population synthesis studies that are interested in the formation of X-ray binaries, the nature and event rate of gravitational wave sources, and answering questions about chemical enrichment.https://resolver.caltech.edu/CaltechAUTHORS:20141216-133549029Directed search for gravitational waves from Scorpius X-1 with initial LIGO data
https://resolver.caltech.edu/CaltechAUTHORS:20150504-084613268
Year: 2015
DOI: 10.1103/PhysRevD.91.062008
We present results of a search for continuously emitted gravitational radiation, directed at the brightest low-mass x-ray binary, Scorpius X-1. Our semicoherent analysis covers 10 days of LIGO S5 data ranging from 50–550 Hz, and performs an incoherent sum of coherent F-statistic power distributed amongst frequency-modulated orbital sidebands. All candidates not removed at the veto stage were found to be consistent with noise at a 1% false alarm rate. We present Bayesian 95% confidence upper limits on gravitational-wave strain amplitude using two different prior distributions: a standard one, with no a priori assumptions about the orientation of Scorpius X-1; and an angle-restricted one, using a prior derived from electromagnetic observations. Median strain upper limits of 1.3×10^(−24) and 8×10^(−25) are reported at 150 Hz for the standard and angle-restricted searches respectively. This proof-of-principle analysis was limited to a short observation time by unknown effects of accretion on the intrinsic spin frequency of the neutron star, but improves upon previous upper limits by factors of ∼1.4 for the standard, and 2.3 for the angle-restricted search at the sensitive region of the detector.https://resolver.caltech.edu/CaltechAUTHORS:20150504-084613268Advanced LIGO
https://resolver.caltech.edu/CaltechAUTHORS:20150417-133457450
Year: 2015
DOI: 10.1088/0264-9381/32/7/074001
The Advanced LIGO gravitational wave detectors are second-generation instruments designed and built for the two LIGO observatories in Hanford, WA and Livingston, LA, USA. The two instruments are identical in design, and are specialized versions of a Michelson interferometer with 4 km long arms. As in Initial LIGO, Fabry–Perot cavities are used in the arms to increase the interaction time with a gravitational wave, and power recycling is used to increase the effective laser power. Signal recycling has been added in Advanced LIGO to improve the frequency response. In the most sensitive frequency region around 100 Hz, the design strain sensitivity is a factor of 10 better than Initial LIGO. In addition, the low frequency end of the sensitivity band is moved from 40 Hz down to 10 Hz. All interferometer components have been replaced with improved technologies to achieve this sensitivity gain. Much better seismic isolation and test mass suspensions are responsible for the gains at lower frequencies. Higher laser power, larger test masses and improved mirror coatings lead to the improved sensitivity at mid and high frequencies. Data collecting runs with these new instruments are planned to begin in mid-2015.https://resolver.caltech.edu/CaltechAUTHORS:20150417-133457450Dark matter-induced collapse of neutron stars: a possible link between fast radio bursts and the missing pulsar problem
https://resolver.caltech.edu/CaltechAUTHORS:20150618-135406893
Year: 2015
DOI: 10.1093/mnrasl/slv049
Fast radio bursts (FRBs) are an emerging class of short and bright radio transients whose sources remain enigmatic. Within the Galactic Centre, the non-detection of pulsars within the inner ∼10 pc has created a missing pulsar problem that has intensified with time. With all reserve, we advance the notion that the two problems could be linked by a common solution: the collapse of neutron stars (NS) due to capture and sedimentation of dark matter (DM) within their cores. Bramante & Linden showed that certain DM properties allow for rapid NS collapse within the high DM density environments near galactic centres while permitting NS survival elsewhere. Each DM-induced collapse could generate an FRB as the NS magnetosphere is suddenly expelled. This scenario could explain several features of FRBs: their short time scales, large energies, locally produced scattering tails, and high event rates. We predict that FRBs are localized to galactic centres, and that our own galactic centre harbours a large population of NS-mass (M ∼ 1.4 M⊙) black holes. The DM-induced collapse scenario is intrinsically unlikely because it can only occur in a small region of allowable DM parameter space. However, if observed to occur, it would place tight constraints on DM properties.https://resolver.caltech.edu/CaltechAUTHORS:20150618-135406893Characterization of the LIGO detectors during their sixth science run
https://resolver.caltech.edu/CaltechAUTHORS:20150622-120844267
Year: 2015
DOI: 10.1088/0264-9381/32/11/115012
In 2009–2010, the Laser Interferometer Gravitational-Wave Observatory (LIGO) operated together with international partners Virgo and GEO600 as a network to search for gravitational waves (GWs) of astrophysical origin. The sensitivity of these detectors was limited by a combination of noise sources inherent to the instrumental design and its environment, often localized in time or frequency, that couple into the GW readout. Here we review the performance of the LIGO instruments during this epoch, the work done to characterize the detectors and their data, and the effect that transient and continuous noise artefacts have on the sensitivity of LIGO to a variety of astrophysical sources.https://resolver.caltech.edu/CaltechAUTHORS:20150622-120844267Post-merger evolution of a neutron star-black hole binary with neutrino transport
https://resolver.caltech.edu/CaltechAUTHORS:20150706-102844751
Year: 2015
DOI: 10.1103/PhysRevD.91.124021
We present a first simulation of the post-merger evolution of a black hole-neutron star binary in full general relativity using an energy-integrated general-relativistic truncated moment formalism for neutrino transport. We describe our implementation of the moment formalism and important tests of our code, before studying the formation phase of an accretion disk after a black hole-neutron star merger. We use as initial data an existing general-relativistic simulation of the merger of a neutron star of mass 1.4M_⊙ with a black hole of mass 7M_⊙ and dimensionless spin χBH=0.8. Comparing with a simpler leakage scheme for the treatment of the neutrinos, we find noticeable differences in the neutron-to-proton ratio in and around the disk, and in the neutrino luminosity. We find that the electron neutrino luminosity is much lower in the transport simulations, and that both the disk and the disk outflows are less neutron rich. The spatial distribution of the neutrinos is significantly affected by relativistic effects, due to large velocities and curvature in the regions of strongest emission. Over the short time scale evolved, we do not observe purely neutrino-driven outflows. However, a small amount of material (3×10^(−4)M_⊙) is ejected in the polar region during the circularization of the disk. Most of that material is ejected early in the formation of the disk, and is fairly neutron rich (electron fraction Ye∼0.15–0.25). Through r-process nucleosynthesis, that material should produce high-opacity lanthanides in the polar region, and could thus affect the light curve of radioactively powered electromagnetic transients. We also show that by the end of the simulation, while the bulk of the disk remains neutron rich (Ye∼0.15–0.2 and decreasing), its outer layers have a higher electron fraction: 10% of the remaining mass has Ye>0.3. As that material would be the first to be unbound by disk outflows on longer time scales, and as composition evolution is slower at later times, the changes in Ye experienced during the formation phase of the disk could have an impact on nucleosynthesis outputs from neutrino-driven and viscously driven outflows. Finally, we find that the effective viscosity due to momentum transport by neutrinos is unlikely to have a strong effect on the growth of the magnetorotational instability in the post-merger accretion disk.https://resolver.caltech.edu/CaltechAUTHORS:20150706-102844751Supernova seismology: gravitational wave signatures of rapidly rotating core collapse
https://resolver.caltech.edu/CaltechAUTHORS:20150715-150545994
Year: 2015
DOI: 10.1093/mnras/stv698
Gravitational waves (GW) generated during a core-collapse supernova open a window into the heart of the explosion. At core bounce, progenitors with rapid core rotation rates exhibit a characteristic GW signal which can be used to constrain the properties of the core of the progenitor star. We investigate the dynamics of rapidly rotating core collapse, focusing on hydrodynamic waves generated by the core bounce, and the GW spectrum they produce. The centrifugal distortion of the rapidly rotating proto-neutron star (PNS) leads to the generation of axisymmetric quadrupolar oscillations within the PNS and surrounding envelope. Using linear perturbation theory, we estimate the frequencies, amplitudes, damping times, and GW spectra of the oscillations. Our analysis provides a qualitative explanation for several features of the GW spectrum and shows reasonable agreement with non-linear hydrodynamic simulations, although a few discrepancies due to non-linear/rotational effects are evident. The dominant early post-bounce GW signal is produced by the fundamental quadrupolar oscillation mode of the PNS, at a frequency 0.70 ≲ f ≲ 0.80 kHz, whose energy is largely trapped within the PNS and leaks out on a ∼10-ms time-scale. Quasi-radial oscillations are not trapped within the PNS and quickly propagate outwards until they steepen into shocks. Both the PNS structure and Coriolis/centrifugal forces have a strong impact on the GW spectrum, and a detection of the GW signal can therefore be used to constrain progenitor properties.https://resolver.caltech.edu/CaltechAUTHORS:20150715-150545994Neutrino-driven Turbulent Convection and Standing Accretion Shock Instability in Three-Dimensional Core-Collapse Supernovae
https://resolver.caltech.edu/CaltechAUTHORS:20141217-093048536
Year: 2015
DOI: 10.1088/0004-637X/808/1/70
We conduct a series of numerical experiments into the nature of three-dimensional (3D) hydrodynamics in the postbounce stalled-shock phase of core-collapse supernovae using 3D general-relativistic hydrodynamic simulations of a 27-M⊙ progenitor star with a neutrino leakage/heating scheme. We vary the strength of neutrino heating and find three cases of 3D dynamics: (1) neutrino-driven convection, (2) initially neutrino-driven convection and subsequent development of the standing accretion shock instability (SASI), (3) SASI dominated evolution. This confirms previous 3D results of Hanke et al. 2013, ApJ 770, 66 and Couch & Connor 2014, ApJ 785, 123. We carry out simulations with resolutions differing by up to a factor of ∼4 and demonstrate that low resolution is artificially favorable for explosion in the 3D convection-dominated case, since it decreases the efficiency of energy transport to small scales. Low resolution results in higher radial convective fluxes of energy and enthalpy, more fully buoyant mass, and stronger neutrino heating. In the SASI-dominated case, lower resolution damps SASI oscillations. In the convection-dominated case, a quasi-stationary angular kinetic energy spectrum E(ℓ) develops in the heating layer. Like other 3D studies, we find E(ℓ)∝ℓ−1 in the "inertial range," while theory and local simulations argue for E(ℓ)∝ℓ−5/3. We argue that current 3D simulations do not resolve the inertial range of turbulence and are affected by numerical viscosity up to the energy containing scale, creating a "bottleneck" that prevents an efficient turbulent cascade.https://resolver.caltech.edu/CaltechAUTHORS:20141217-093048536Implicit large eddy simulations of anisotropic weakly compressible turbulence with application to core-collapse supernovae
https://resolver.caltech.edu/CaltechAUTHORS:20160404-142032802
Year: 2015
DOI: 10.1186/s40668-015-0011-0
In the implicit large eddy simulation (ILES) paradigm, the dissipative nature of high-resolution shock-capturing schemes is exploited to provide an implicit model of turbulence. The ILES approach has been applied to different contexts, with varying degrees of success. It is the de-facto standard in many astrophysical simulations and in particular in studies of core-collapse supernovae (CCSN). Recent 3D simulations suggest that turbulence might play a crucial role in core-collapse supernova explosions, however the fidelity with which turbulence is simulated in these studies is unclear. Especially considering that the accuracy of ILES for the regime of interest in CCSN, weakly compressible and strongly anisotropic, has not been systematically assessed before. Anisotropy, in particular, could impact the dissipative properties of the flow and enhance the turbulent pressure in the radial direction, favouring the explosion. In this paper we assess the accuracy of ILES using numerical methods most commonly employed in computational astrophysics by means of a number of local simulations of driven, weakly compressible, anisotropic turbulence. Our simulations employ several different methods and span a wide range of resolutions. We report a detailed analysis of the way in which the turbulent cascade is influenced by the numerics. Our results suggest that anisotropy and compressibility in CCSN turbulence have little effect on the turbulent kinetic energy spectrum and a Kolmogorov k^(-5/3) scaling is obtained in the inertial range. We find that, on the one hand, the kinetic energy dissipation rate at large scales is correctly captured even at low resolutions, suggesting that very high "effective Reynolds number" can be achieved at the largest scales of the simulation. On the other hand, the dynamics at intermediate scales appears to be completely dominated by the so-called bottleneck effect, i.e., the pile up of kinetic energy close to the dissipation range due to the partial suppression of the energy cascade by numerical viscosity. An inertial range is not recovered until the point where high resolution ~ 512^3, which would be difficult to realize in global simulations, is reached. We discuss the consequences for CCSN simulations.https://resolver.caltech.edu/CaltechAUTHORS:20160404-142032802Searches for Continuous Gravitational Waves from Nine Young Supernova Remnants
https://resolver.caltech.edu/CaltechAUTHORS:20151218-125808925
Year: 2015
DOI: 10.1088/0004-637X/813/1/39
We describe directed searches for continuous gravitational waves (GWs) in data from the sixth Laser Interferometer Gravitational-wave Observatory (LIGO) science data run. The targets were nine young supernova remnants not associated with pulsars; eight of the remnants are associated with non-pulsing suspected neutron stars. One target's parameters are uncertain enough to warrant two searches, for a total of 10. Each search covered a broad band of frequencies and first and second frequency derivatives for a fixed sky direction. The searches coherently integrated data from the two LIGO interferometers over time spans from 5.3–25.3 days using the matched-filtering F-statistic. We found no evidence of GW signals. We set 95% confidence upper limits as strong (low) as 4 × 10^(−25) on intrinsic strain, 2 × 10^(−7) on fiducial ellipticity, and 4 × 10^(−5) on r-mode amplitude. These beat the indirect limits from energy conservation and are within the range of theoretical predictions for neutron-star ellipticities and r-mode amplitudes.https://resolver.caltech.edu/CaltechAUTHORS:20151218-125808925Monte Carlo Neutrino Transport Through Remnant Disks from Neutron Star Mergers
https://resolver.caltech.edu/CaltechAUTHORS:20150803-071008836
Year: 2015
DOI: 10.1088/0004-637X/813/1/38
We present Sedonu, a new open source, steady-state, special relativistic Monte Carlo (MC) neutrino transport code, available at bitbucket.org/srichers/sedonu. The code calculates the energy- and angle-dependent neutrino distribution function on fluid backgrounds of any number of spatial dimensions, calculates the rates of change of fluid internal energy and electron fraction, and solves for the equilibrium fluid temperature and electron fraction. We apply this method to snapshots from two-dimensional simulations of accretion disks left behind by binary neutron star mergers, varying the input physics and comparing to the results obtained with a leakage scheme for the cases of a central black hole and a central hypermassive neutron star. Neutrinos are guided away from the densest regions of the disk and escape preferentially around 45° from the equatorial plane. Neutrino heating is strengthened by MC transport a few scale heights above the disk midplane near the innermost stable circular orbit, potentially leading to a stronger neutrino-driven wind. Neutrino cooling in the dense midplane of the disk is stronger when using MC transport, leading to a globally higher cooling rate by a factor of a few and a larger leptonization rate by an order of magnitude. We calculate neutrino pair annihilation rates and estimate that an energy of 2.8 × 10^(46) erg is deposited within 45° of the symmetry axis over 300 ms when a central BH is present. Similarly, 1.9 × 10^(48) erg is deposited over 3 s when an HMNS sits at the center, but neither estimate is likely to be sufficient to drive a gamma-ray burst jet.https://resolver.caltech.edu/CaltechAUTHORS:20150803-071008836Light Curves of Core-Collapse Supernovae with Substantial Mass Loss using the New Open-Source SuperNova Explosion Code (SNEC)
https://resolver.caltech.edu/CaltechAUTHORS:20150615-054321068
Year: 2015
DOI: 10.1088/0004-637X/814/1/63
We present the SuperNova Explosion Code (SNEC), an open-source Lagrangian code for the hydrodynamics
and equilibrium-diffusion radiation transport in the expanding envelopes of supernovae. Given a model of a
progenitor star, an explosion energy, and an amount and distribution of radioactive nickel, SNEC generates the
bolometric light curve, as well as the light curves in different wavelength bands assuming black body emission.
As a first application of SNEC, we consider the explosions of a grid of 15 M_⊙ (at zero-age main sequence)
stars whose hydrogen envelopes are stripped to different extents and at different points in their evolution. The
resulting light curves exhibit plateaus with durations of ∼20 − 100 days if & 1.5 − 2 M_⊙ of hydrogen-rich
material is left and no plateau if less hydrogen-rich material is left. The shorter plateau lengths are unlike the
Type IIP supernova light curves typically observed in nature. This suggests that, at least for zero-age main
sequence masses . 20 M_⊙, hydrogen mass loss occurs as an all or nothing process, perhaps pointing to the
important role binary interactions play in observed mass-stripped supernovae (i.e., Type Ib/c events). These
light curves are also unlike what is typically seen for Type IIL supernovae, arguing that simply varying the
amount of mass loss cannot explain these events. The most stripped models begin to show double-peaked light
curves similar to what is often seen for Type IIb supernovae, confirming previous work that these supernovae
can come from progenitors that have a small amount of hydrogen and a radius of ∼ 500 R_⊙.https://resolver.caltech.edu/CaltechAUTHORS:20150615-054321068A large scale dynamo and magnetoturbulence in rapidly rotating core-collapse supernovae
https://resolver.caltech.edu/CaltechAUTHORS:20150914-145343070
Year: 2015
DOI: 10.1038/nature15755
Magnetohydrodynamic turbulence is important in many high-energy astrophysical systems, where instabilities can amplify the local magnetic field over very short timescales. Specifically, the magnetorotational instability and dynamo action have been suggested as a mechanism for the growth of magnetar-strength magnetic fields (of 10^(15) gauss and above) and for powering the explosion of a rotating massive star. Such stars are candidate progenitors of type Ic-bl hypernovae, which make up all supernovae that are connected to long γ-ray bursts. The magnetorotational instability has been studied with local high-resolution shearing-box simulations in three dimensions, and with global two-dimensional simulations, but it is not known whether turbulence driven by this instability can result in the creation of a large-scale, ordered and dynamically relevant field. Here we report results from global, three-dimensional, general-relativistic magnetohydrodynamic turbulence simulations. We show that hydromagnetic turbulence in rapidly rotating protoneutron stars produces an inverse cascade of energy. We find a large-scale, ordered toroidal field that is consistent with the formation of bipolar magnetorotationally driven outflows. Our results demonstrate that rapidly rotating massive stars are plausible progenitors for both type Ic-bl supernovae and long γ-ray bursts, and provide a viable mechanism for the formation of magnetars. Moreover, our findings suggest that rapidly rotating massive stars might lie behind potentially magnetar-powered superluminous supernovae.https://resolver.caltech.edu/CaltechAUTHORS:20150914-145343070Prospects for Observing and Localizing Gravitational-Wave Transients with Advanced LIGO and Advanced Virgo
https://resolver.caltech.edu/CaltechAUTHORS:20160314-070823106
Year: 2016
DOI: 10.1007/lrr-2016-1
PMCID: PMC5256041
We present a possible observing scenario for the Advanced LIGO and Advanced Virgo gravitational-wave detectors over the next decade, with the intention of providing information to the astronomy community to facilitate planning for multi-messenger astronomy with gravitational waves. We determine the expected sensitivity of the network to transient gravitational-wave signals, and study the capability of the network to determine the sky location of the source. We report our findings for gravitational-wave transients, with particular focus on gravitational-wave signals from the inspiral of binary neutron-star systems, which are considered the most promising for multi-messenger astronomy. The ability to localize the sources of the detected signals depends on the geographical distribution of the detectors and their relative sensitivity, and 90% credible regions can be as large as thousands of square degrees when only two sensitive detectors are operational. Determining the sky position of a significant fraction of detected signals to areas of 5 deg^2 to 20 deg^2 will require at least three detectors of sensitivity within a factor of ∼ 2 of each other and with a broad frequency bandwidth. Should the third LIGO detector be relocated to India as expected, a significant fraction of gravitational-wave signals will be localized to a few square degrees by gravitational-wave observations alone.https://resolver.caltech.edu/CaltechAUTHORS:20160314-070823106Observation of Gravitational Waves from a Binary Black Hole Merger
https://resolver.caltech.edu/CaltechAUTHORS:20160211-080913893
Year: 2016
DOI: 10.1103/PhysRevLett.116.061102
On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-Wave Observatory simultaneously observed a transient gravitational-wave signal. The signal sweeps upwards in frequency from 35 to 250 Hz with a peak gravitational-wave strain of 1.0×10^(−21). It matches the waveform predicted by general relativity for the inspiral and merger of a pair of black holes and the ringdown of the resulting single black hole. The signal was observed with a matched-filter signal-to-noise ratio of 24 and a false alarm rate estimated to be less than 1 event per 203 000 years, equivalent to a significance greater than 5.1σ. The source lies at a luminosity distance of 410 +160/−180 Mpc corresponding to a redshift z=0.09 +0.03/−0.04. In the source frame, the initial black hole masses are 36+5−4M_⊙ and 29+4−4M_⊙, and the final black hole mass is 62+4−4M⊙, with 3.0+0.5−0.5M_⊙c^2 radiated in gravitational waves. All uncertainties define 90% credible intervals. These observations demonstrate the existence of binary stellar-mass black hole systems. This is the first direct detection of gravitational waves and the first observation of a binary black hole merger.https://resolver.caltech.edu/CaltechAUTHORS:20160211-080913893Gravitational waveforms for neutron star binaries from binary black hole simulations
https://resolver.caltech.edu/CaltechAUTHORS:20160119-152925358
Year: 2016
DOI: 10.1103/PhysRevD.93.044064
Gravitational waves from binary neutron star (BNS) and black-hole/neutron star (BHNS) inspirals are primary sources for detection by the Advanced Laser Interferometer Gravitational-Wave Observatory. The tidal forces acting on the neutron stars induce changes in the phase evolution of
the gravitational waveform, and these changes can be used to constrain the nuclear equation of state. Current methods of generating BNS and BHNS waveforms rely on either computationally challenging full 3D hydrodynamical simulations or approximate analytic solutions. We introduce a new method for computing inspiral waveforms for BNS/BHNS systems by adding the post-Newtonian (PN) tidal effects to full numerical simulations of binary black holes (BBHs), effectively replacing the non-tidal terms in the PN expansion with BBH results. Comparing a waveform generated with this method against a full hydrodynamical simulation of a BNS inspiral yields a phase difference of < 1 radian over ~ 15 orbits. The numerical phase accuracy required of BNS simulations to measure the accuracy of the method we present here is estimated as a function of the tidal deformability parameter ⋋.https://resolver.caltech.edu/CaltechAUTHORS:20160119-152925358Observing gravitational waves from core-collapse supernovae in the advanced detector era
https://resolver.caltech.edu/CaltechAUTHORS:20160303-142154512
Year: 2016
DOI: 10.1103/PhysRevD.93.042002
The next galactic core-collapse supernova (CCSN) has already exploded, and its electromagnetic (EM) waves, neutrinos, and gravitational waves (GWs) may arrive at any moment. We present an extensive study on the potential sensitivity of prospective detection scenarios for GWs from CCSNe within 5 Mpc, using realistic noise at the predicted sensitivity of the Advanced LIGO and Advanced Virgo detectors for 2015, 2017, and 2019. We quantify the detectability of GWs from CCSNe within the Milky Way and Large Magellanic Cloud, for which there will be an observed neutrino burst. We also consider extreme GW emission scenarios for more distant CCSNe with an associated EM signature. We find that a three-detector network at design sensitivity will be able to detect neutrino-driven CCSN explosions out to ∼5.5 kpc, while rapidly rotating core collapse will be detectable out to the Large Magellanic Cloud at 50 kpc. Of the phenomenological models for extreme GW emission scenarios considered in this study, such as long-lived bar-mode instabilities and disk fragmentation instabilities, all models considered will be detectable out to M31 at 0.77 Mpc, while the most extreme models will be detectable out to M82 at 3.52 Mpc and beyond.https://resolver.caltech.edu/CaltechAUTHORS:20160303-142154512All-sky search for long-duration gravitational wave transients with initial LIGO
https://resolver.caltech.edu/CaltechAUTHORS:20160316-082945850
Year: 2016
DOI: 10.1103/PhysRevD.93.042005
We present the results of a search for long-duration gravitational wave transients in two sets of data collected by the LIGO Hanford and LIGO Livingston detectors between November 5, 2005 and September 30, 2007, and July 7, 2009 and October 20, 2010, with a total observational time of 283.0 days and 132.9 days, respectively. The search targets gravitational wave transients of duration 10–500 s in a frequency band of 40–1000 Hz, with minimal assumptions about the signal waveform, polarization, source direction, or time of occurrence. All candidate triggers were consistent with the expected background; as a result we set 90% confidence upper limits on the rate of long-duration gravitational wave transients for different types of gravitational wave signals. For signals from black hole accretion disk instabilities, we set upper limits on the source rate density between 3.4×10^(−5) and 9.4×10^(−4) Mpc^(−3) yr^(−1) at 90% confidence. These are the first results from an all-sky search for unmodeled long-duration transient gravitational waves.https://resolver.caltech.edu/CaltechAUTHORS:20160316-082945850Low mass binary neutron star mergers: Gravitational waves and neutrino emission
https://resolver.caltech.edu/CaltechAUTHORS:20160226-081934022
Year: 2016
DOI: 10.1103/PhysRevD.93.044019
Neutron star mergers are among the most promising sources of gravitational waves for advanced ground-based detectors. These mergers are also expected to power bright electromagnetic signals, in the form of short gamma-ray bursts, infrared/optical transients powered by r-process nucleosynthesis in neutron-rich material ejected by the merger, and radio emission from the interaction of that ejecta with the interstellar medium. Simulations of these mergers with fully general relativistic codes are critical to understand the merger and postmerger gravitational wave signals and their neutrinos and electromagnetic counterparts. In this paper, we employ the Spectral Einstein Code to simulate the merger of low mass neutron star binaries (two 1.2M⊙ neutron stars) for a set of three nuclear-theory-based, finite temperature equations of state. We show that the frequency peaks of the postmerger gravitational wave signal are in good agreement with predictions obtained from recent simulations using a simpler treatment of gravity. We find, however, that only the fundamental mode of the remnant is excited for long periods of time: emission at the secondary peaks is damped on a millisecond time scale in the simulated binaries. For such low mass systems, the remnant is a massive neutron star which, depending on the equation of state, is either permanently stable or long lived (i.e. rapid uniform rotation is sufficient to prevent its collapse). We observe strong excitations of l=2, m=2 modes, both in the massive neutron star and in the form of hot, shocked tidal arms in the surrounding accretion torus. We estimate the neutrino emission of the remnant using a neutrino leakage scheme and, in one case, compare these results with a gray two-moment neutrino transport scheme. We confirm the complex geometry of the neutrino emission, also observed in previous simulations with neutrino leakage, and show explicitly the presence of important differences in the neutrino luminosity, disk composition, and outflow properties between the neutrino leakage and transport schemes.https://resolver.caltech.edu/CaltechAUTHORS:20160226-081934022First low frequency all-sky search for continuous gravitational wave signals
https://resolver.caltech.edu/CaltechAUTHORS:20160301-082544147
Year: 2016
DOI: 10.1103/PhysRevD.93.042007
In this paper we present the results of the first low frequency all-sky search of continuous gravitational wave signals conducted on Virgo VSR2 and VSR4 data. The search covered the full sky, a frequency range between 20 Hz and 128 Hz with a range of spin-down between −1.0×10^(−10) Hz/s and +1.5×10^(−11) Hz/s, and was based on a hierarchical approach. The starting point was a set of short Fast Fourier Transforms (FFT), of length 8192 seconds, built from the calibrated strain data. Aggressive data cleaning, both in the time and frequency domains, has been done in order to remove, as much as possible, the effect of disturbances of instrumental origin. On each dataset a number of candidates has been selected, using the FrequencyHough transform in an incoherent step. Only coincident candidates among VSR2 and VSR4 have been examined in order to strongly reduce the false alarm probability, and the most significant candidates have been selected. The criteria we have used for candidate selection and for the coincidence step greatly reduce the harmful effect of large instrumental artifacts. Selected candidates have been subject to a follow-up by constructing a new set of longer FFTs followed by a further incoherent analysis. No evidence for continuous gravitational wave signals was found, therefore we have set a population-based joint VSR2-VSR4 90% confidence level upper limit on the dimensionless gravitational wave strain in the frequency range between 20 Hz and 128 Hz. This is the first all-sky search for continuous gravitational waves conducted at frequencies below 50 Hz. We set upper limits in the range between about 10^(−24) and 2×10^(−23) at most frequencies. Our upper limits on signal strain show an improvement of up to a factor of ∼2 with respect to the results of previous all-sky searches at frequencies below 80 Hz.https://resolver.caltech.edu/CaltechAUTHORS:20160301-082544147Astrophysical Implications of the Binary Black Hole Merger GW150914
https://resolver.caltech.edu/CaltechAUTHORS:20160315-110429411
Year: 2016
DOI: 10.3847/2041-8205/818/2/L22
The discovery of the gravitational-wave (GW) source GW150914 with the Advanced LIGO detectors provides the first observational evidence for the existence of binary black hole (BH) systems that inspiral and merge within the age of the universe. Such BH mergers have been predicted in two main types of formation models, involving isolated binaries in galactic fields or dynamical interactions in young and old dense stellar environments. The measured masses robustly demonstrate that relatively "heavy" BHs (≳25 M_☉) can form in nature. This discovery implies relatively weak massive-star winds and thus the formation of GW150914 in an environment with a metallicity lower than about 1/2 of the solar value. The rate of binary-BH (BBH) mergers inferred from the observation of GW150914 is consistent with the higher end of rate predictions (≳1 Gpc^(−3) yr^(−1)) from both types of formation models. The low measured redshift (z ≃ 0.1) of GW150914 and the low inferred metallicity of the stellar progenitor imply either BBH formation in a low-mass galaxy in the local universe and a prompt merger, or formation at high redshift with a time delay between formation and merger of several Gyr. This discovery motivates further studies of binary-BH formation astrophysics. It also has implications for future detections and studies by Advanced LIGO and Advanced Virgo, and GW detectors in space.https://resolver.caltech.edu/CaltechAUTHORS:20160315-110429411Neutrino-Driven Convection in Core-Collapse Supernovae: High-Resolution Simulations
https://resolver.caltech.edu/CaltechAUTHORS:20160307-101746459
Year: 2016
DOI: 10.3847/0004-637X/820/1/76
We present results from high-resolution semiglobal simulations of neutrino-driven convection in core-collapse supernovae. We employ an idealized setup with parametrized neutrino heating/cooling and nuclear dissociation at the shock front. We study the internal dynamics of neutrino-driven convection and its role in re-distributing energy and momentum through the gain region. We find that even if buoyant plumes are able to locally transfer heat up to the shock, convection is not able to create a net positive energy flux and overcome the downwards transport of energy from the accretion flow. Turbulent convection does, however, provide a significant effective pressure support to the accretion flow as it favors the accumulation of energy, mass and momentum in the gain region. We derive an approximate equation that is able to explain and predict the shock evolution in terms of integrals of quantities such as the turbulent pressure in the gain region or the effects of nonradial motion of the fluid. We use this relation as a way to quantify the role of turbulence in the dynamics of the accretion shock. Finally, we investigate the effects of grid resolution, which we change by a factor 20 between the lowest and highest resolution. Our results show that the shallow slopes of the turbulent kinetic energy spectra reported in previous studies are a numerical artefact. Kolmogorov scaling is progressively recovered as the resolution is increased.https://resolver.caltech.edu/CaltechAUTHORS:20160307-101746459Numerical simulations of stellar collapse in scalar-tensor theories of gravity
https://resolver.caltech.edu/CaltechAUTHORS:20160718-103943012
Year: 2016
DOI: 10.1088/0264-9381/33/13/135002
We present numerical-relativity simulations of spherically symmetric core collapse and compact-object formation in scalar-tensor theories of gravity. The additional scalar degree of freedom introduces a propagating monopole gravitational-wave mode. Detection of monopole scalar waves with current and future gravitational-wave experiments may constitute smoking gun evidence for strong-field modifications of general relativity. We collapse both polytropic and more realistic pre-supernova profiles using a high-resolution shock-capturing scheme and an approximate prescription for the nuclear equation of state. The most promising sources of scalar radiation are protoneutron stars collapsing to black holes. In case of a galactic core collapse event forming a black hole, Advanced LIGO may be able to place independent constraints on the parameters of the theory at a level comparable to current solar-system and binary-pulsar measurements. In the region of the parameter space admitting spontaneously scalarised stars, transition to configurations with prominent scalar hair before black-hole formation further enhances the emitted signal. Although a more realistic treatment of the microphysics is necessary to fully investigate the occurrence of spontaneous scalarisation of neutron star remnants, we speculate that formation of such objects could constrain the parameters of the theory beyond the current bounds obtained with solar-system and binary-pulsar experiments.https://resolver.caltech.edu/CaltechAUTHORS:20160718-103943012Observing gravitational-wave transient GW150914 with minimal assumptions
https://resolver.caltech.edu/CaltechAUTHORS:20160609-132951028
Year: 2016
DOI: 10.1103/PhysRevD.93.122004
The gravitational-wave signal GW150914 was first identified on September 14, 2015, by searches for short-duration gravitational-wave transients. These searches identify time-correlated transients in multiple detectors with minimal assumptions about the signal morphology, allowing them to be sensitive to gravitational waves emitted by a wide range of sources including binary black hole mergers. Over the observational period from September 12 to October 20, 2015, these transient searches were sensitive to binary black hole mergers similar to GW150914 to an average distance of ∼600 Mpc. In this paper, we describe the analyses that first detected GW150914 as well as the parameter estimation and waveform reconstruction techniques that initially identified GW150914 as the merger of two black holes. We find that the reconstructed waveform is consistent with the signal from a binary black hole merger with a chirp mass of ∼30 M_⊙ and a total mass before merger of ∼70 M_⊙ in the detector frame.https://resolver.caltech.edu/CaltechAUTHORS:20160609-132951028High-energy Neutrino follow-up search of Gravitational Wave Event GW150914 with ANTARES and IceCube
https://resolver.caltech.edu/CaltechAUTHORS:20160628-092040474
Year: 2016
DOI: 10.1103/PhysRevD.93.122010
We present the high-energy-neutrino follow-up observations of the first gravitational wave transient GW150914 observed by the Advanced LIGO detectors on September 14, 2015. We search for coincident neutrino candidates within the data recorded by the IceCube and Antares neutrino detectors. A possible joint detection could be used in targeted electromagnetic follow-up observations, given the significantly better angular resolution of neutrino events compared to gravitational waves. We find no neutrino candidates in both temporal and spatial coincidence with the gravitational wave event. Within ±500 s of the gravitational wave event, the number of neutrino candidates detected by IceCube and Antares were three and zero, respectively. This is consistent with the expected atmospheric background, and none of the neutrino candidates were directionally coincident with GW150914. We use this nondetection to constrain neutrino emission from the gravitational-wave event.https://resolver.caltech.edu/CaltechAUTHORS:20160628-092040474Simulations of inspiraling and merging double neutron stars using the Spectral Einstein Code
https://resolver.caltech.edu/CaltechAUTHORS:20160627-150704335
Year: 2016
DOI: 10.1103/PhysRevD.93.124062
We present results on the inspiral, merger, and postmerger evolution of a neutron star-neutron star (NSNS) system. Our results are obtained using the hybrid pseudospectral-finite volume Spectral Einstein Code (SpEC). To test our numerical methods, we evolve an equal-mass system for ≈22 orbits before merger. This waveform is the longest waveform obtained from fully general-relativistic simulations for NSNSs to date. Such long (and accurate) numerical waveforms are required to further improve semianalytical models used in gravitational wave data analysis, for example, the effective one body models. We discuss in detail the improvements to SpEC's ability to simulate NSNS mergers, in particular mesh refined grids to better resolve the merger and postmerger phases. We provide a set of consistency checks and compare our results to NSNS merger simulations with the independent bam code. We find agreement between them, which increases confidence in results obtained with either code. This work paves the way for future studies using long waveforms and more complex microphysical descriptions of neutron star matter in SpEC.https://resolver.caltech.edu/CaltechAUTHORS:20160627-150704335How loud are neutron star mergers?
https://resolver.caltech.edu/CaltechAUTHORS:20160729-143411841
Year: 2016
DOI: 10.1103/PhysRevD.94.024023
We present results from the first large parameter study of neutron star mergers using fully general relativistic simulations with finite-temperature microphysical equations of state and neutrino cooling. We consider equal and unequal-mass binaries drawn from the galactic population and simulate each binary with three different equations of state. Our focus is on the emission of energy and angular momentum in gravitational waves in the postmerger phase. We find that the emitted gravitational-wave energy in the first ∼10 ms of the life of the resulting hypermassive neutron star (HMNS) is about twice the energy emitted over the entire inspiral history of the binary. The total radiated energy per binary mass is comparable to or larger than that of nonspinning black hole inspiral-mergers. About 0.8–2.5% of the binary mass-energy is emitted at kHz frequencies in the early HMNS evolution. We find a clear dependence of the postmerger gravitational wave emission on binary configuration and equation of state and show that it can be encoded as a broad function of the binary tidal coupling constant κ^T_2. Our results also demonstrate that the dimensionless spin of black holes resulting from subsequent HMNS collapse are limited to ≲ 0.7–0.8. This may significantly impact the neutrino pair annihilation mechanism for powering short gamma-ray bursts (sGRB).https://resolver.caltech.edu/CaltechAUTHORS:20160729-143411841Dynamical Mass Ejection from Binary Neutron Star Mergers
https://resolver.caltech.edu/CaltechAUTHORS:20160912-101351820
Year: 2016
DOI: 10.1093/mnras/stw1227
We present fully general-relativistic simulations of binary neutron star mergers with a temperature and composition dependent nuclear equation of state. We study the dynamical mass ejection from both quasi-circular and dynamical-capture eccentric mergers. We systematically vary the level of our treatment of the microphysics to isolate the effects of neutrino cooling and heating and we compute the nucleosynthetic yields of the ejecta. We find that eccentric binaries can eject significantly more material than quasi-circular binaries and generate bright infrared and radio emission. In all our simulations the outflow is composed of a combination of tidally- and shock-driven ejecta, mostly distributed over a broad ∼60∘ angle from the orbital plane, and, to a lesser extent, by thermally driven winds at high latitudes. Ejecta from eccentric mergers are typically more neutron rich than those of quasi-circular mergers. We find neutrino cooling and heating to affect, quantitatively and qualitatively, composition, morphology, and total mass of the outflows. This is also reflected in the infrared and radio signatures of the binary. The final nucleosynthetic yields of the ejecta are robust and insensitive to input physics or merger type in the regions of the second and third r-process peaks. The yields for elements on the first peak vary between our simulations, but none of our models is able to explain the Solar abundances of first-peak elements without invoking additional first-peak contributions from either neutrino and viscously-driven winds operating on longer timescales after the mergers, or from core-collapse supernovae.https://resolver.caltech.edu/CaltechAUTHORS:20160912-101351820Massive Computation for Understanding Core-Collapse Supernova Explosions
https://resolver.caltech.edu/CaltechAUTHORS:20160830-065811875
Year: 2016
DOI: 10.1109/MCSE.2016.81
How do massive stars explode? Progress toward the answer is driven by increases in compute power. Petascale supercomputers are enabling detailed 3D simulations of core-collapse supernovae that are elucidating the role of fluid instabilities, turbulence, and magnetic field amplification in supernova engines.https://resolver.caltech.edu/CaltechAUTHORS:20160830-065811875One-armed spiral instability in neutron star mergers and its detectability in gravitational waves
https://resolver.caltech.edu/CaltechAUTHORS:20160906-113920827
Year: 2016
DOI: 10.1103/PhysRevD.94.064011
We study the development and saturation of the m=1 one-armed spiral instability in remnants of binary neutron star mergers by means of high-resolution long-term numerical relativity simulations. Our results suggest that this instability is a generic outcome of neutron star mergers in astrophysically relevant configurations, including both "stiff" and "soft" nuclear equations of state. We find that, once seeded at merger, the m=1 mode saturates within ∼10 ms and persists over secular time scales. Gravitational waves emitted by the m=1 instability have a peak frequency around 1–2 kHz and, if detected, they could be used to constrain the equation of state of neutron stars. We construct hybrid waveforms spanning the entire Advanced LIGO band by combining our high-resolution numerical data with state-of-the-art effective-one-body waveforms including tidal effects. We use the complete hybrid waveforms to study the detectability of the one-armed spiral instability for both Advanced LIGO and the Einstein Telescope. We conclude that the one-armed spiral instability is not an efficient gravitational wave emitter. Even under very optimistic assumptions, Advanced LIGO will only be able to detect the one-armed instability up to ∼3 Mpc, which corresponds to an event rate of 10^(−7) yr^(−1) to 10^(−4) yr^(−1). Third-generation detectors or better will likely be required to observe the one-armed instability.https://resolver.caltech.edu/CaltechAUTHORS:20160906-113920827Numerical Modeling of the Early Light Curves of Type IIP Supernovae
https://resolver.caltech.edu/CaltechAUTHORS:20160928-152014062
Year: 2016
DOI: 10.3847/0004-637X/829/2/109
The early rise of Type IIP supernovae (SN IIP) provides important information for constraining the properties of their progenitors. This can, in turn, be compared to pre-explosion imaging constraints and stellar models to develop a more complete picture of how massive stars evolve and end their lives. Using the SuperNova Explosion Code (SNEC), we model the first 40 days of SNe IIP to better understand what constraints can be derived from their early light curves. We use two sets of red supergiant (RSG) progenitor models with zero-age main sequence masses in the range between 9 M⊙ and 20 M⊙. We find that the early properties of the light curve depend most sensitively on the radius of the progenitor, and thus provide a relation between the g-band rise time and the radius at the time of explosion. This relation will be useful for deriving constraints on progenitors from future observations, especially in cases where detailed modeling of the entire rise is not practical. When comparing to observed rise times, the radii we find are a factor of a few larger than previous semi-analytic derivations and are generally in better agreement with what is found with current stellar evolution calculations as well as direct observations of RSGs.https://resolver.caltech.edu/CaltechAUTHORS:20160928-152014062General-Relativistic Three-Dimensional Multi-group Neutrino Radiation-Hydrodynamics Simulations of Core-Collapse Supernovae
https://resolver.caltech.edu/CaltechAUTHORS:20161028-100923991
Year: 2016
DOI: 10.3847/0004-637X/831/1/98
We report on a set of long-term general-relativistic three-dimensional (3D) multi-group (energy-dependent) neutrino radiation-hydrodynamics simulations of core-collapse supernovae. We employ a full 3D two-moment scheme with the local M1 closure, three neutrino species, and 12 energy groups per species. With this, we follow the post-core-bounce evolution of the core of a nonrotating 27-M⊙ progenitor in full unconstrained 3D and in octant symmetry for ≳380 ms. We find the development of an asymmetric runaway explosion in our unconstrained simulation. We test the resolution dependence of our results and, in agreement with previous work, find that low resolution artificially aids explosion and leads to an earlier runaway expansion of the shock. At low resolution, the octant and full 3D dynamics are qualitatively very similar, but at high resolution, only the full 3D simulation exhibits the onset of explosion.https://resolver.caltech.edu/CaltechAUTHORS:20161028-100923991A First Targeted Search for Gravitational-Wave Bursts from Core-Collapse Supernovae in Data of First-Generation Laser Interferometer Detectors
https://resolver.caltech.edu/CaltechAUTHORS:20161004-100308686
Year: 2016
DOI: 10.1103/PhysRevD.94.102001
We present results from a search for gravitational-wave bursts coincident with two core-collapse supernovae observed optically in 2007 and 2011. We employ data from the Laser Interferometer Gravitational-wave Observatory (LIGO), the Virgo gravitational-wave observatory, and the GEO 600 gravitational-wave observatory. The targeted core-collapse supernovae were selected on the basis of (1) proximity (within approximately 15 Mpc), (2) tightness of observational constraints on the time of core collapse that defines the gravitational-wave search window, and (3) coincident operation of at least two interferometers at the time of core collapse.We find no plausible gravitational-wave candidates. We present the probability of detecting signals from both astrophysically well-motivated and more speculative gravitational-wave emission mechanisms as a function of distance from Earth, and discuss the implications for the detection of gravitational waves from core-collapse supernovae by the upgraded Advanced LIGO and Virgo detectors.https://resolver.caltech.edu/CaltechAUTHORS:20161004-100308686The Influence of Neutrinos on r-Process Nucleosynthesis in the Ejecta of Black Hole-Neutron Star Mergers
https://resolver.caltech.edu/CaltechAUTHORS:20161010-133838091
Year: 2017
DOI: 10.1093/mnras/stw2622
During the merger of a black hole and a neutron star, baryonic mass can become unbound from the system. Because the ejected material is extremely neutron-rich, the r-process rapidly synthesizes heavy nuclides as the material expands and cools. In this work, we map general relativistic models of black hole–neutron star mergers into a Newtonian smoothed particle hydrodynamics (SPH) code and follow the evolution of the thermodynamics and morphology of the ejecta until the outflows become homologous. We investigate how the subsequent evolution depends on our mapping procedure and find that the results are robust. Using thermodynamic histories from the SPH particles, we then calculate the expected nucleosynthesis in these outflows while varying the level of neutrino irradiation coming from the post-merger accretion disc. We find that the ejected material robustly produces r-process nucleosynthesis even for unrealistically high neutrino luminosities, due to the rapid velocities of the outflow. None the less, we find that neutrinos can have an impact on the detailed pattern of the r-process nucleosynthesis. Electron neutrinos are captured by neutrons to produce protons while neutron capture is occurring. The produced protons rapidly form low-mass seed nuclei for the r-process. These low-mass seeds are eventually incorporated into the first r-process peak at A ∼ 78. We consider the mechanism of this process in detail and discuss if it can impact galactic chemical evolution of the first peak r-process nuclei.https://resolver.caltech.edu/CaltechAUTHORS:20161010-133838091Equation of State Effects on Gravitational Waves from Rotating Core Collapse
https://resolver.caltech.edu/CaltechAUTHORS:20170201-102929694
Year: 2017
DOI: 10.1103/PhysRevD.95.063019
Gravitational waves (GWs) generated by axisymmetric rotating collapse, bounce, and early postbounce phases of a galactic core-collapse supernova are detectable by current-generation gravitational wave observatories. Since these GWs are emitted from the quadrupole-deformed nuclear-density core, they may encode information on the uncertain nuclear equation of state (EOS). We examine the effects of the nuclear EOS on GWs from rotating core collapse and carry out 1824 axisymmetric general-relativistic hydrodynamic simulations that cover a parameter space of 98 different rotation profiles and 18 different EOS. We show that the bounce GW signal is largely independent of the EOS and sensitive primarily to the ratio of rotational to gravitational energy, T/|W|, and at high rotation rates, to the degree of differential rotation. The GW frequency (f_(peak)∼600–1000 Hz) of postbounce core oscillations shows stronger EOS dependence that can be parametrized by the core's EOS-dependent dynamical frequency √Gρc. We find that the ratio of the peak frequency to the dynamical frequency f_(peak)/√Gρc follows a universal trend that is obeyed by all EOS and rotation profiles and that indicates that the nature of the core oscillations changes when the rotation rate exceeds the dynamical frequency. We find that differences in the treatments of low-density nonuniform nuclear matter, of the transition from nonuniform to uniform nuclear matter, and in the description of nuclear matter up to around twice saturation density can mildly affect the GW signal. More exotic, higher-density physics is not probed by GWs from rotating core collapse. We furthermore test the sensitivity of the GW signal to variations in the treatment of nuclear electron capture during collapse. We find that approximations and uncertainties in electron capture rates can lead to variations in the GW signal that are of comparable magnitude to those due to different nuclear EOS. This emphasizes the need for reliable experimental and/or theoretical nuclear electron capture rates and for self-consistent multidimensional neutrino radiation-hydrodynamic simulations of rotating core collapse.https://resolver.caltech.edu/CaltechAUTHORS:20170201-102929694Calibration of the Advanced LIGO detectors for the discovery of the binary black-hole merger GW150914
https://resolver.caltech.edu/CaltechAUTHORS:20161018-154345711
Year: 2017
DOI: 10.1103/PhysRevD.95.062003
In Advanced LIGO, detection and astrophysical source parameter estimation of the binary black hole merger GW150914 requires a calibrated estimate of the gravitational-wave strain sensed by the detectors. Producing an estimate from each detector's differential arm length control loop readout signals requires applying time domain filters, which are designed from a frequency domain model of the detector's gravitational-wave response. The gravitational-wave response model is determined by the detector's opto-mechanical response and the properties of its feedback control system. The measurements used to validate the model and characterize its uncertainty are derived primarily from a dedicated photon radiation pressure actuator, with cross-checks provided by optical and radio frequency references. We describe how the gravitational-wave readout signal is calibrated into equivalent gravitational-wave-induced strain and how the statistical uncertainties and systematic errors are assessed. Detector data collected over 38 calendar days, from September 12 to October 20, 2015, contain the event GW150914 and approximately 16 days of coincident data used to estimate the event false alarm probability. The calibration uncertainty is less than 10% in magnitude and 10° in phase across the relevant frequency band, 20 Hz to 1 kHz.https://resolver.caltech.edu/CaltechAUTHORS:20161018-154345711SpECTRE: A task-based discontinuous Galerkin code for relativistic astrophysics
https://resolver.caltech.edu/CaltechAUTHORS:20170427-142549072
Year: 2017
DOI: 10.1016/j.jcp.2016.12.059
We introduce a new relativistic astrophysics code, SpECTRE, that combines a discontinuous Galerkin method with a task-based parallelism model. SpECTRE's goal is to achieve more accurate solutions for challenging relativistic astrophysics problems such as core-collapse supernovae and binary neutron star mergers. The robustness of the discontinuous Galerkin method allows for the use of high-resolution shock capturing methods in regions where (relativistic) shocks are found, while exploiting high-order accuracy in smooth regions. A task-based parallelism model allows efficient use of the largest supercomputers for problems with a heterogeneous workload over disparate spatial and temporal scales. We argue that the locality and algorithmic structure of discontinuous Galerkin methods will exhibit good scalability within a task-based parallelism framework. We demonstrate the code on a wide variety of challenging benchmark problems in (non)-relativistic (magneto)-hydrodynamics. We demonstrate the code's scalability including its strong scaling on the NCSA Blue Waters supercomputer up to the machine's full capacity of 22,380 nodes using 671,400 threads.https://resolver.caltech.edu/CaltechAUTHORS:20170427-142549072Probing Extreme-density Matter with Gravitational-wave Observations of Binary Neutron Star Merger Remnants
https://resolver.caltech.edu/CaltechAUTHORS:20170616-100047138
Year: 2017
DOI: 10.3847/2041-8213/aa775f
We present a proof-of-concept study, based on numerical-relativity simulations, of how gravitational waves (GWs) from neutron star merger remnants can probe the nature of matter at extreme densities. Phase transitions and extra degrees of freedom can emerge at densities beyond those reached during the inspiral, and typically result in a softening of the equation of state (EOS). We show that such physical effects change the qualitative dynamics of the remnant evolution, but they are not identifiable as a signature in the GW frequency, with the exception of possible black hole formation effects. The EOS softening is, instead, encoded in the GW luminosity and phase and is in principle detectable up to distances of the order of several megaparsecs with advanced detectors and up to hundreds of megaparsecs with third-generation detectors. Probing extreme-density matter will require going beyond the current paradigm and developing a more holistic strategy for modeling and analyzing postmerger GW signals.https://resolver.caltech.edu/CaltechAUTHORS:20170616-100047138Systematic survey of the effects of wind mass loss algorithms on the evolution of single massive stars
https://resolver.caltech.edu/CaltechAUTHORS:20170901-125621395
Year: 2017
DOI: 10.1051/0004-6361/201730698
Mass loss processes are a key uncertainty in the evolution of massive stars. They determine the amount of mass and angular momentum retained by the star, thus influencing its evolution and presupernova structure. Because of the high complexity of the physical processes driving mass loss, stellar evolution calculations must employ parametric algorithms, and usually only include wind mass loss. We carried out an extensive parameter study of wind mass loss and its effects on massive star evolution using the open-source stellar evolution code MESA. We provide a systematic comparison of wind mass loss algorithms for solar-metallicity, nonrotating, single stars in the initial mass range of 15 M⊙ to 35 M⊙. We consider combinations drawn from two hot phase (i.e., roughly the main sequence) algorithms, three cool phase (i.e., post-main-sequence) algorithms, and two Wolf-Rayet mass loss algorithms. We discuss separately the effects of mass loss in each of these phases. In addition, we consider linear wind efficiency scale factors of 1, 0.33, and 0.1 to account for suggested reductions in mass loss rates due to wind inhomogeneities. We find that the initial to final mass mapping for each zero-age main-sequence (ZAMS) mass has a ~ 50% uncertainty if all algorithm combinations and wind efficiencies are considered. The ad-hoc efficiency scale factor dominates this uncertainty. While the final total mass and internal structure of our models vary tremendously with mass loss treatment, final luminosity and effective temperature are much less sensitive for stars with ZAMS mass ≲ 30 M⊙. This indicates that uncertainty in wind mass loss does not negatively affect estimates of the ZAMS mass of most single-star supernova progenitors from pre-explosion observations. Our results furthermore show that the internal structure of presupernova stars is sensitive to variations in both main sequence and post main-sequence mass loss. The compactness parameter ξ ∝ ℳ /R(ℳ) has been identified as a proxy for the "explodability" of a given presupernova model. We find that ξ varies by as much as 30% for models of the same ZAMS mass evolved with different wind efficiencies and mass loss algorithm combinations. This suggests that the details of the mass loss treatment might bias the outcome of detailed core-collapse supernova calculations and the predictions for neutron star and black hole formation.https://resolver.caltech.edu/CaltechAUTHORS:20170901-125621395Numerical relativity waveform surrogate model for generically precessing binary black hole mergers
https://resolver.caltech.edu/CaltechAUTHORS:20170801-103324737
Year: 2017
DOI: 10.1103/PhysRevD.96.024058
A generic, noneccentric binary black hole (BBH) system emits gravitational waves (GWs) that are completely described by seven intrinsic parameters: the black hole spin vectors and the ratio of their masses. Simulating a BBH coalescence by solving Einstein's equations numerically is computationally expensive, requiring days to months of computing resources for a single set of parameter values. Since theoretical predictions of the GWs are often needed for many different source parameters, a fast and accurate model is essential. We present the first surrogate model for GWs from the coalescence of BBHs including all seven dimensions of the intrinsic noneccentric parameter space. The surrogate model, which we call NRSur7dq2, is built from the results of 744 numerical relativity simulations. NRSur7dq2 covers spin magnitudes up to 0.8 and mass ratios up to 2, includes all ℓ≤4 modes, begins about 20 orbits before merger, and can be evaluated in ∼50 ms. We find the largest NRSur7dq2 errors to be comparable to the largest errors in the numerical relativity simulations, and more than an order of magnitude smaller than the errors of other waveform models. Our model, and more broadly the methods developed here, will enable studies that were not previously possible when using highly accurate waveforms, such as parameter inference and tests of general relativity with GW observations.https://resolver.caltech.edu/CaltechAUTHORS:20170801-103324737A Detailed Comparison of Multidimensional Boltzmann Neutrino Transport Methods in Core-collapse Supernovae
https://resolver.caltech.edu/CaltechAUTHORS:20171003-130753873
Year: 2017
DOI: 10.3847/1538-4357/aa8bb2
The mechanism driving core-collapse supernovae is sensitive to the interplay between matter and neutrino radiation. However, neutrino radiation transport is very difficult to simulate, and several radiation transport methods of varying levels of approximation are available. We carefully compare for the first time in multiple spatial dimensions the discrete ordinates (DO) code of Nagakura, Yamada, and Sumiyoshi and the Monte Carlo (MC) code Sedonu, under the assumptions of a static fluid background, flat spacetime, elastic scattering, and full special relativity. We find remarkably good agreement in all spectral, angular, and fluid interaction quantities, lending confidence to both methods. The DO method excels in determining the heating and cooling rates in the optically thick region. The MC method predicts sharper angular features due to the effectively infinite angular resolution, but struggles to drive down noise in quantities where subtractive cancellation is prevalent, such as the net gain in the protoneutron star and off-diagonal components of the Eddington tensor. We also find that errors in the angular moments of the distribution functions induced by neglecting velocity dependence are subdominant to those from limited momentum-space resolution. We briefly compare directly computed second angular moments to those predicted by popular algebraic two-moment closures, and we find that the errors from the approximate closures are comparable to the difference between the DO and MC methods. Included in this work is an improved Sedonu code, which now implements a fully special relativistic, time-independent version of the grid-agnostic MC random walk approximation.https://resolver.caltech.edu/CaltechAUTHORS:20171003-130753873Gravitational Waves from Binary Black Hole Mergers Inside of Stars
https://resolver.caltech.edu/CaltechAUTHORS:20170718-135919962
Year: 2017
DOI: 10.1103/PhysRevLett.119.171103
We present results from a controlled numerical experiment investigating the effect of stellar density gas on the coalescence of binary black holes (BBHs) and the resulting gravitational waves (GWs). This investigation is motivated by the proposed stellar core fragmentation scenario for BBH formation and the associated possibility of an electromagnetic counterpart to a BBH GW event. We employ full numerical relativity coupled with general-relativistic hydrodynamics and set up a 30+30 M⊙ BBH (motivated by GW150914) inside gas with realistic stellar densities. Our results show that at densities ρ≳10^6–10^7 g cm^(−3) dynamical friction between the BHs and gas changes the coalescence dynamics and the GW signal in an unmistakable way. We show that for GW150914, LIGO observations appear to rule out BBH coalescence inside stellar gas of ρ≳10^7 g cm^(−3). Typical densities in the collapsing cores of massive stars are in excess of this density. This excludes the fragmentation scenario for the formation of GW150914.https://resolver.caltech.edu/CaltechAUTHORS:20170718-135919962Long-Lived Inverse Chirp Signals from Core-Collapse in Massive Scalar-Tensor Gravity
https://resolver.caltech.edu/CaltechAUTHORS:20171120-101916041
Year: 2017
DOI: 10.1103/PhysRevLett.119.201103
This Letter considers stellar core collapse in massive scalar-tensor theories of gravity. The presence of a mass term for the scalar field allows for dramatic increases in the radiated gravitational wave signal. There are several potential smoking gun signatures of a departure from general relativity associated with this process. These signatures could show up within existing LIGO-Virgo searches.https://resolver.caltech.edu/CaltechAUTHORS:20171120-101916041Signatures of hypermassive neutron star lifetimes on r-process nucleosynthesis in the disc ejecta from neutron star mergers
https://resolver.caltech.edu/CaltechAUTHORS:20171117-125429012
Year: 2017
DOI: 10.1093/mnras/stx1987
We investigate the nucleosynthesis of heavy elements in the winds ejected by accretion discs formed in neutron star mergers. We compute the element formation in disc outflows from hypermassive neutron star (HMNS) remnants of variable lifetime, including the effect of angular momentum transport in the disc evolution. We employ long-term axisymmetric hydrodynamic disc simulations to model the ejecta, and compute r-process nucleosynthesis with tracer particles using a nuclear reaction network containing ∼8000 species. We find that the previously known strong correlation between HMNS lifetime, ejected mass and average electron fraction in the outflow is directly related to the amount of neutrino irradiation on the disc, which dominates mass ejection at early times in the form of a neutrino-driven wind. Production of lanthanides and actinides saturates at short HMNS lifetimes (≲10 ms), with additional ejecta contributing to a blue optical kilonova component for longer-lived HMNSs. We find good agreement between the abundances from the disc outflow alone and the solar r-process distribution only for short HMNS lifetimes (≲10 ms). For longer lifetimes, the rare-earth and third r-process peaks are significantly underproduced compared to the solar pattern, requiring additional contributions from the dynamical ejecta. The nucleosynthesis signature from a spinning black hole (BH) can only overlap with that from an HMNS of moderate lifetime (≲60 ms). Finally, we show that angular momentum transport not only contributes with a late-time outflow component, but that it also enhances the neutrino-driven component by moving material to shallower regions of the gravitational potential, in addition to providing additional heating.https://resolver.caltech.edu/CaltechAUTHORS:20171117-125429012Open-source nuclear equation of state framework based on the liquid-drop model with Skyrme interaction
https://resolver.caltech.edu/CaltechAUTHORS:20170713-154611153
Year: 2017
DOI: 10.1103/PhysRevC.96.065802
The equation of state (EOS) of dense matter is an essential ingredient for numerical simulations of core-collapse supernovae and neutron star mergers. The properties of matter near and above nuclear saturation density are uncertain, which translates into uncertainties in astrophysical simulations and their multimessenger signatures. Therefore, a wide range of EOSs spanning the allowed range of nuclear interactions are necessary for determining the sensitivity of these astrophysical phenomena and their signatures to variations in input microphysics. We present a new set of finite temperature EOSs based on experimentally allowed Skyrme forces. We employ a liquid-drop model of nuclei to capture the nonuniform phase of nuclear matter at subsaturation density, which is blended into a nuclear statistical equilibrium EOS at lower densities. We also provide a new, open-source code for calculating EOSs for arbitrary Skyrme parametrizations. We then study the effects of different Skyrme parametrizations on thermodynamical properties of dense astrophysical matter, the neutron star mass-radius relationship, and the core collapse of 15 and 40 solar mass stars.https://resolver.caltech.edu/CaltechAUTHORS:20170713-154611153The Progenitor Dependence of Core-collapse Supernovae from Three-dimensional Simulations with Progenitor Models of 12–40 M_⊙
https://resolver.caltech.edu/CaltechAUTHORS:20180226-150225175
Year: 2018
DOI: 10.3847/2041-8213/aaa967
We present a first study of the progenitor star dependence of the three-dimensional (3D) neutrino mechanism of core-collapse supernovae. We employ full 3D general-relativistic multi-group neutrino radiation-hydrodynamics and simulate the postbounce evolutions of progenitors with zero-age main sequence masses of 12, 15, 20, 27, and 40 M_⊙. All progenitors, with the exception of the 12 M_⊙ star, experience shock runaway by the end of their simulations. In most cases, a strongly asymmetric explosion will result. We find three qualitatively distinct evolutions that suggest a complex dependence of explosion dynamics on progenitor density structure, neutrino heating, and 3D flow. (1) Progenitors with massive cores, shallow density profiles, and high post-core-bounce accretion rates experience very strong neutrino heating and neutrino-driven turbulent convection, leading to early shock runaway. Accretion continues at a high rate, likely leading to black hole formation. (2) Intermediate progenitors experience neutrino-driven, turbulence-aided explosions triggered by the arrival of density discontinuities at the shock. These occur typically at the silicon/silicon–oxygen shell boundary. (3) Progenitors with small cores and density profiles without strong discontinuities experience shock recession and develop the 3D standing-accretion shock instability (SASI). Shock runaway ensues late, once declining accretion rate, SASI, and neutrino-driven convection create favorable conditions. These differences in explosion times and dynamics result in a non-monotonic relationship between progenitor and compact remnant mass.https://resolver.caltech.edu/CaltechAUTHORS:20180226-150225175Evolution of the magnetized, neutrino-cooled accretion disk in the aftermath of a black hole-neutron star binary merger
https://resolver.caltech.edu/CaltechAUTHORS:20180430-101115120
Year: 2018
DOI: 10.1103/PhysRevD.97.083014
Black hole–torus systems from compact binary mergers are possible engines for gamma-ray bursts (GRBs). During the early evolution of the postmerger remnant, the state of the torus is determined by a combination of neutrino cooling and magnetically driven heating processes, so realistic models must include both effects. In this paper, we study the postmerger evolution of a magnetized black hole–neutron star binary system using the Spectral Einstein Code (SpEC) from an initial postmerger state provided by previous numerical relativity simulations. We use a finite-temperature nuclear equation of state and incorporate neutrino effects in a leakage approximation. To achieve the needed accuracy, we introduce improvements to SpEC's implementation of general-relativistic magnetohydrodynamics (MHD), including the use of cubed-sphere multipatch grids and an improved method for dealing with supersonic accretion flows where primitive variable recovery is difficult. We find that a seed magnetic field triggers a sustained source of heating, but its thermal effects are largely cancelled by the accretion and spreading of the torus from MHD-related angular momentum transport. The neutrino luminosity peaks at the start of the simulation, and then drops significantly over the first 20 ms but in roughly the same way for magnetized and nonmagnetized disks. The heating rate and disk's luminosity decrease much more slowly thereafter. These features of the evolution are insensitive to grid structure and resolution, formulation of the MHD equations, and seed field strength, although turbulent effects are not fully converged.https://resolver.caltech.edu/CaltechAUTHORS:20180430-101115120Turbulence in core-collapse supernovae
https://resolver.caltech.edu/CaltechAUTHORS:20180409-101015330
Year: 2018
DOI: 10.1088/1361-6471/aab872
Multidimensional simulations show that non-radial, turbulent, fluid motion is a fundamental component of the core-collapse supernova explosion mechanism. Neutrino-driven convection, the standing accretion shock instability, and relic-perturbations from advanced nuclear burning stages can all impact the outcome of core collapse in a qualitative and quantitative way. Here, we review the current understanding of these phenomena and their role in the explosion of massive stars. We also discuss the role of protoneutron star convection and of magnetic fields in the context of the delayed neutrino mechanism.https://resolver.caltech.edu/CaltechAUTHORS:20180409-101015330Low-mass X-ray binaries from black hole retaining globular clusters
https://resolver.caltech.edu/CaltechAUTHORS:20180627-144419500
Year: 2018
DOI: 10.1093/mnras/sty659
Recent studies suggest that globular clusters (GCs) may retain a substantial population of stellar-mass black holes (BHs), in contrast to the long-held belief of a few to zero BHs. We model the population of BH low-mass X-ray binaries (BH-LMXBs), an ideal observable proxy for elusive single BHs, produced from a representative group of Milky Way GCs with variable BH populations. We simulate the formation of BH binaries in GCs through exchange interactions between binary and single stars in the company of tens to hundreds of BHs. Additionally, we consider the impact of the BH population on the rate of compact binaries undergoing gravitational wave driven mergers. The characteristics of the BH-LMXB population and binary properties are sensitive to the GCs structural parameters as well as its unobservable BH population. We find that GCs retaining ∼1000 BHs produce a galactic population of ∼150 ejected BH-LMXBs, whereas GCs retaining only ∼20 BHs produce zero ejected BH-LMXBs. Moreover, we explore the possibility that some of the presently known BH-LMXBs might have originated in GCs and identify five candidate systems.https://resolver.caltech.edu/CaltechAUTHORS:20180627-144419500r-process Nucleosynthesis from Three-dimensional Magnetorotational Core-collapse Supernovae
https://resolver.caltech.edu/CaltechAUTHORS:20180926-131800822
Year: 2018
DOI: 10.3847/1538-4357/aad6ec
We investigate r-process nucleosynthesis in 3D general-relativistic magnetohydrodynamic simulations of rapidly rotating strongly magnetized core collapse. The simulations include a microphysical finite-temperature equation of state and a leakage scheme that captures the overall energetics and lepton number exchange due to postbounce neutrino emission and absorption. We track the composition of the ejected material using the nuclear reaction network SkyNet. Our results show that the 3D dynamics of magnetorotational core-collapse supernovae (CCSN) are important for their nucleosynthetic signature. We find that production of r-process material beyond the second peak is reduced by a factor of 100 when the magnetorotational jets produced by the rapidly rotating core undergo a kink instability. Our results indicate that 3D magnetorotationally powered CCSNe are robust r-process sources only if they are obtained by the collapse of cores with unrealistically large precollapse magnetic fields of the order of 10^(13) G. Additionally, a comparison simulation that we restrict to axisymmetry results in overly optimistic r-process production for lower magnetic field strengths.https://resolver.caltech.edu/CaltechAUTHORS:20180926-131800822Equation of state effects in the core collapse of a
20−M_⊙ star
https://resolver.caltech.edu/CaltechAUTHORS:20191107-104247204
Year: 2019
DOI: 10.1103/physrevc.100.055802
Uncertainties in our knowledge of the properties of dense matter near and above nuclear saturation density are among the main sources of variations in multimessenger signatures predicted for core-collapse supernovae (CCSNe) and the properties of neutron stars (NSs). We construct 97 new finite-temperature equations of state (EOSs) of dense matter that obey current experimental, observational, and theoretical constraints and discuss how systematic variations in the EOS parameters affect the properties of cold nonrotating NSs and the core collapse of a
20−M_⊙ progenitor star. The core collapse of the 20−M_⊙ progenitor star is simulated in spherical symmetry using the general-relativistic radiation-hydrodynamics code GR1D where neutrino interactions are computed for each EOS using the NuLib library. We conclude that the effective mass of nucleons at densities above nuclear saturation density is the largest source of uncertainty in the CCSN neutrino signal and dynamics even though it plays a subdominant role in most properties of cold NS matter. Meanwhile, changes in other observables affect the properties of cold NSs, while having little effect in CCSNe. To strengthen our conclusions, we perform six octant three-dimensional CCSN simulations varying the effective mass of nucleons at nuclear saturation density. We conclude that neutrino heating and, thus, the likelihood of explosion is significantly increased for EOSs where the effective mass of nucleons at nuclear saturation density is large.https://resolver.caltech.edu/CaltechAUTHORS:20191107-104247204Core collapse in massive scalar-tensor gravity
https://resolver.caltech.edu/CaltechAUTHORS:20200807-090347626
Year: 2020
DOI: 10.1103/physrevd.102.044010
This paper provides an extended exploration of the inverse-chirp gravitational-wave signals from stellar collapse in massive scalar-tensor gravity reported in [Phys. Rev. Lett. 119, 201103]. We systematically explore the parameter space that characterizes the progenitor stars, the equation of state, and the scalar-tensor theory of the core collapse events. We identify a remarkably simple and straightforward classification scheme of the resulting collapse events. For any given set of parameters, the collapse leads to one of three end states: a weakly scalarized neutron star, a strongly scalarized neutron star, or a black hole, possibly formed in multiple stages. The latter two end states can lead to strong gravitational-wave signals that may be detectable in present continuous-wave searches with ground-based detectors. We identify a very sharp boundary in the parameter space that separates events with strong gravitational-wave emission from those with negligible radiation.https://resolver.caltech.edu/CaltechAUTHORS:20200807-090347626