@phdthesis {CaltechTHESIS_https://thesis.library.caltech.edu/id/eprint/14184, title ="Probing the Inner Accretion Flow Properties Around Black Holes with X-ray Observations", author = "Xu, Yanjun", month = "January", year = "2021", url = "https://resolver.caltech.edu/CaltechTHESIS:05272021-232230617", revision_no = "42", abstract = "

Accretion, governed by gravity, is a fundamental source of energy in the universe. Accretion is important to the growth and evolution of black holes, as well as the structure in the universe on larger scales. Accretion disks around supermassive black holes are likely to have produced most of the ionizing radiation in the universe since the epoch of reionization. Outflows launched from the accretion disk of black holes, either in the form of disk winds or jets, provide a feedback mechanism that plays an important role in the co-evolution of black holes and their host galaxies. It has been decades since the fundamental theories about black hole accretion were established. Black hole accretion has been widely studied in active galactic nuclei (AGNs) and black hole binaries ever since the early days of X-ray astronomy. However, important questions still remain regarding the fundamental physical properties of black holes, the structure and geometry of accretion disks and coronae, and the nature of disk winds and jets.

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In this thesis, I report results from recent X-ray observations of an ultra-luminous infrared galaxy with a central AGN, IRAS 05189–2524, and several black hole X-ray binaries (or black hole candidates for those currently lacking dynamical mass determinations), IGR J17091–3624, MAXI J1535–571, Swift J1658.2–4242, MAXI J1631–479, and MAXI J1820+070. Most of the black hole X-ray binaries studied in this thesis were uncatalogued sources and were discovered as bright Galactic X-ray transients over the past few years, offering great opportunities for investigating black hole accretion with high quality datasets. The launch of the NuSTAR telescope in 2012 has brought the advanced capabilities of performing high sensitivity observations in the hard X-ray band and remaining free from pile-up for bright Galactic sources, providing new angles for the detailed study of observational phenomena around both supermassive and stellar-mass accreting black holes. I study the inner accretion flow properties around black holes in the above objects by conducting spectral and timing analyses of the NuSTAR observations and the simultaneous soft X-ray band data from the Swift or XMM-Newton telescope. I have searched for and analyzed various accretion related observational features in these systems (disk reflection spectra, ionized absorption caused by disk winds, quasi periodic oscillations (QPOs) in the X-ray light curves, and unusual aperiodic flux variations or accretion state changes), and interpret the results in terms of the physical properties about the inner accretion flows and the central black holes.

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By modeling the relativistic disk reflection spectra, I have found that the inner edge of the optically-thick accretion disk is truncated in IGR J17091–3624 during the bright hard state, in MAXI J1631–479 during the very high state, and in MAXI J1820+070 during the faint hard state, whereas the inner accretion disk is consistent with extending down to the ISCO in MAXI J1535–571 and Swift J1658.2–4242 during their bright hard states, and in MAXI J1631–479 during its soft state. Based on all the observational evidence gathered in this thesis, the general picture about the accretion flow geometry at different accretion states seems to be more complicated than that from previously well accepted theoretical predictions. In addition, I have measured the black holes spins and inner accretion disk inclinations for IRAS 05189–2524, MAXI J1535–571, Swift J1658.2–4242, and MAXI J1631–479 using the disk reflection modeling method, where the central black holes are all found to be rapidly spinning. There are also evidence for strong disk winds detected in the X-ray spectra of some of the objects studied in this thesis with interesting implications. I summarize my studies of these individual objects at the end of this thesis in a more broader context of the characteristic behaviors of the population and their general physical implications, and discuss about the possibilities of extending my research in this direction with the upcoming new X-ray missions.

", } @phdthesis {CaltechTHESIS_https://thesis.library.caltech.edu/id/eprint/11168, title ="Voyager 1 Observations of Galactic Cosmic Ray Anisotropies in the Local Interstellar Medium", author = "Rankin, Jamie Sue", month = "January", year = "2019", doi = "10.7907/WX3S-1D86", url = "https://resolver.caltech.edu/CaltechTHESIS:09022018-124448728", revision_no = "28", abstract = "Since crossing the heliopause on August 25, 2012, Voyager 1 has observed reductions in galactic cosmic ray counting rates caused by a time-varying depletion of particles with pitch angles near 90-deg, while intensities of particles with other pitch angles remain unchanged. Between late 2012 and mid-2017, three large-scale, durable events occurred, lasting roughly 100 to 630 days. Omnidirectional and directional data from the Cosmic Ray Subsystem's high energy telescopes are used to report observations of the cosmic ray intensity variations. Omnidirectional (>20 MeV) proton-dominated measurements show up to a ~3.8% intensity reduction. Bi-directional (>70 MeV) and unidirectional (~18 to ~70 MeV) proton-dominated measurements are analyzed using data taken from various spacecraft orientations, including during magnetometer roll calibrations and 70-deg-offset maneuvers. The anisotropy is characterized as a \"notch\" in an otherwise uniform pitch-angle distribution of varying depth and width centered about 90-deg in pitch angle space. The notch averages ~22-deg wide and ~15% deep -- signifying a depletion region that is broad and shallow. However, electron observations reveal that there is only a weak, at most, evidence of pitch angle anisotropy in cosmic-ray electrons with energies of ~3 to ~105 MeV, indicating that the generation of the notch or its evolution differs between electrons and protons, or varies with rigidity. There are indications that the anisotropy is formed by a combination of magnetic trapping and adiabatic cooling in associated shocks or compression regions.\r\n", } @phdthesis {CaltechTHESIS_https://thesis.library.caltech.edu/id/eprint/10294, title ="Unveiling the Structure of Active Galactic Nuclei with Hard X-ray Spectroscopy", author = "Balokovic, Mislav", month = "January", year = "2017", url = "https://resolver.caltech.edu/CaltechTHESIS:06052017-232351957", revision_no = "28", abstract = "

Despite the long history of studies of active galactic nuclei (AGN), details of the structure of the accretion flow onto supermassive black holes are far from clear. Work presented in this thesis is directed at unveiling properties of AGN structure through broadband X-ray spectroscopy, with particular emphasis on the hard X-ray band (photon energies above 10 keV). With its unprecedented sensitivity in this energy band, the NuSTAR telescope provides the key observational diagnostics of the properties of the AGN X-ray source, the corona, and the surrounding gas in the accretion disk, the broad-line region, and the torus.

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The first study presented in this thesis focuses on measurements of the optical depth and the temperature of the plasma in the corona of an obscured AGN. Fitting theoretical spectral models for coronal emission to the NuSTAR data constrained these two basic physical parameters under the assumption of either spherical or disk-like geometry for the corona.

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The remainder of the thesis is dedicated to studies of the anisotropic obscuring structure broadly referred to as the torus. One of them is a case study of three heavily obscured AGN with spectra dominated by the X-ray light scattered and reprocessed in the torus, where it is possible to constrain one of the basic torus properties -- its globally averaged column density. The following study presents the calculation of a new spectral model for reprocessing of the intrinsic X-ray continuum within the torus. Its added flexibility compared to previously available models allows for both the average column density of the torus and its covering factor to be constrained from broadband X-ray spectra of a wide variety of AGN.

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The final part of the thesis in based on a large survey of the local obscured AGN population performed with NuSTAR. Spectral modeling of more than a hundred individual AGN, including both old and new spectral models, is presented. From analyses of the X-ray data for a large and representative AGN sample, for the first time, it is found that their tori preferentially have high covering factors and average column densities close to unity optical depth for Compton scattering.

\r\n", } @phdthesis {CaltechTHESIS_https://thesis.library.caltech.edu/id/eprint/8031, title ="Beyond the Blur: Construction and Characterization of the First Autonomous AO System and an AO Survev of Magnetar Proper Motions", author = "Tendulkar, Shriharsh Prakash", month = "January", year = "2014", url = "https://resolver.caltech.edu/CaltechTHESIS:11192013-181748112", revision_no = "41", abstract = "

Adaptive optics (AO) corrects distortions created by atmospheric turbulence and delivers diffraction-limited images on ground-based telescopes. The vastly improved spatial resolution and sensitivity has been utilized for studying everything from the magnetic fields of sunspots upto the internal dynamics of high-redshift galaxies. This thesis about AO science from small and large telescopes is divided into two parts: Robo-AO and magnetar kinematics.

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In the first part, I discuss the construction and performance of the world’s first fully autonomous visible light AO system, Robo-AO, at the Palomar 60-inch telescope. Robo-AO operates extremely efficiently with an overhead < 50s, typically observing about 22 targets every hour. We have performed large AO programs observing a total of over 7,500 targets since May 2012. In the visible band, the images have a Strehl ratio of about 10% and achieve a contrast of upto 6 magnitudes at a separation of 1′′. The full-width at half maximum achieved is 110–130 milli-arcsecond. I describe how Robo-AO is used to constrain the evolutionary models of low-mass pre-main-sequence stars by measuring resolved spectral energy distributions of stellar multiples in the visible band, more than doubling the current sample. I conclude this part with a discussion of possible future improvements to the Robo-AO system.

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In the second part, I describe a study of magnetar kinematics using high-resolution near-infrared (NIR) AO imaging from the 10-meter Keck II telescope. Measuring the proper motions of five magnetars with a precision of upto 0.7 milli-arcsecond/yr, we have more than tripled the previously known sample of magnetar proper motions and proved that magnetar kinematics are equivalent to those of radio pulsars. We conclusively showed that SGR 1900+14 and SGR 1806-20 were ejected from the stellar clusters with which they were traditionally associated. The inferred kinematic ages of these two magnetars are 6±1.8 kyr and 650±300 yr respectively. These ages are a factor of three to four times greater than their respective characteristic ages. The calculated braking index is close to unity as compared to three for the vacuum dipole model and 2.5-2.8 as measured for young pulsars. I conclude this section by describing a search for NIR counterparts of new magnetars and a future promise of polarimetric investigation of a magnetars’ NIR emission mechanism.

", } @phdthesis {CaltechTHESIS_https://thesis.library.caltech.edu/id/eprint/2746, title ="The Energetics and Environments of Swift Gamma-Ray Bursts", author = "Cenko, Stephen Bradley", month = "January", year = "2009", doi = "10.7907/SNRD-BR66", url = "https://resolver.caltech.edu/CaltechETD:etd-06272008-153145", revision_no = "18", abstract = "

For their short durations, gamma-ray bursts (GRBs) are the most electromagnetically luminous objects in the universe. In this thesis, I uses the fascinating objects, both as signposts, indicating the presence of an ultra-relativistic (Lorentz factor ~ 100-1000) outflow from a newly born stellar mass black hole, and as lighthouses, illuminating the circumburst (r ~ 1 pc) and inter-stellar (r ~ 1 kpc) media along the line of sight.

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In Part I, I describe my efforts to automate the Palomar 60 inch telescope (P60), the primary instrument on which much of this thesis is based. Designed to capitalize on NASA's recently launched Swift Gamma-Ray Burst Explorer, P60 now routinely provides moderately fast (t <~ 3 min) and sustained (R <~ 23 mag) observations of GRB afterglows and other optical transients.

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Part II focuses on the geometry and energetics of some of the best-sampled events in the Swift era. I find both GRB050820A and GRB060418 are an order of magnitude more energetic than pre-Swift events, with a total energy release in excess of 10^52 erg. Both GRBs are therefore members of an emerging class of \"hyper-energetic\" GRBs, suggesting a much broader energy distribution than previously thought and challenging current massive star progenitor models.

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Finally, in Part III I study the environments of long-duration GRBs. The massive star progenitors should leave an imprint on the GRB environment, both on the pc scale as a wind from mass loss of the outer envelope, and on the kpc scale from the dense, dusty disk where massive stars form. Interestingly, I demonstrate that GRB070125 exploded in a halo environment, suggesting at least some massive stars form far away from the disk of their host. By comparing the X-ray and optical light curves from a sample of 29 GRBs, I find nearly half show evidence for suppressed optical emission. I attribute this to dust absorption in the host galaxy, consistent with a massive star origin for long-duration GRBs.

", } @phdthesis {CaltechTHESIS_https://thesis.library.caltech.edu/id/eprint/1428, title ="Development of Hard X-ray Imaging Detectors for the High Energy Focusing Telescope", author = "Chen, Chi Ming Hubert", month = "January", year = "2008", doi = "10.7907/A2DS-N675", url = "https://resolver.caltech.edu/CaltechETD:etd-04202008-135222", revision_no = "17", abstract = "

The High Energy Focusing Telescope (HEFT) is a balloon-borne instrument carrying one of the first focusing telescopes for the hard X-ray band (20–70 keV). It makes use of tungsten-silicon multilayer coatings to extend the reflectivity of nested grazing-incidence mirrors beyond 10 keV. It also carries novel semiconductor pixel detectors on its focal planes to match the capabilities of the multilayer mirrors. After a decade of research and development, we achieve with HEFT an angular resolution of 1.5 arcminutes in half-power diameter, and an energy resolution of 1.0 keV full width at half maximum at 60 keV. We launched HEFT for a 25-hour balloon flight in May, 2005; the instrument performed within specification, and observed Cyg X-1, the Crab Nebula, and other celestial hard X-ray sources.

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In this thesis, I lay out the scientific motivations for HEFT, and give an overview of the experiment. I report on our detector development effort in depth, and document the balloon flight of 2005. I also describe a study of two relic radio sources, 0917+75 and 1401−33, with data from the soft X-ray XMM-Newton observatory. With a hard X-ray focusing telescope like HEFT, one can improve the sensitivity and extend the scope of such studies to other classes of objects.

\r\n", } @phdthesis {CaltechTHESIS_https://thesis.library.caltech.edu/id/eprint/2508, title ="Measurements of X-Ray Selected AGN and Novel Superconducting X-Ray Detectors", author = "Eckart, Megan Elizabeth", month = "January", year = "2007", doi = "10.7907/SZAB-0Y07", url = "https://resolver.caltech.edu/CaltechETD:etd-06072007-130804", revision_no = "10", abstract = "

Major astrophysical advances typically come through combining new observational approaches with new technologies. This thesis involves work on both fronts, combining observational work using data from the Chandra X-ray Observatory and Keck Observatory with novel superconducting detector development to further technology for future observatories.

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The subject of the first part of this thesis is the Serendipitous Extragalactic X-ray Source Identification (SEXSI) program, a survey using Chandra data selected to probe the dominant contributors to the 2-10 keV cosmic X-ray background. SEXSI covers more than 2 square degrees of sky and employs optical photometric and spectroscopic followup of sources discovered in archival Chandra fields. The resulting sample consists of 1034 hard X-ray-selected sources with R-band optical-followup imaging, and optical spectroscopy for 477 of the sources, filling the gap between wide-area, shallow surveys and the deep, pencil-beam surveys. The vast majority of the 2-10 keV-selected sample are AGN with redshifts between 0.1 and 3. We discuss results from our survey, including the spectroscopic properties of hard X-ray sources and the relationship between X-ray and optical properties of our sources. In addition, we present infrared data from the Spitzer Space Telescope that cover a subset of the Chandra fields, which allows us to explore the relative strengths of Chandra and Spitzer as black-hole finders.

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The second part of this thesis focuses on microwave kinetic inductance detectors (MKIDs), a superconducting detector technology that has breakthrough potential for providing megapixel imagers with several eV energy resolution for use in future X-ray missions. These detectors utilize simple, thin-film lithographed microwave resonators as photon detectors in a multiplexed readout approach. X-ray absorption in a superconductor creates quasiparticle excitations, with number proportional to the X-ray energy. The surface impedance of a superconductor changes with the quasiparticle density, and if operated at T << Tc, extremely small changes in the surface impedance can be measured using the thin-film resonant circuit and microwave readout techniques. This provides a sensitive detector with excellent energy resolution.

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MKIDs offer the advantage over many other cryogenic detector technologies that they can be easily multiplexed by coupling many resonators to a single microwave transmission line. In addition, the readout electronics can be operated at room temperature, a significant advantage for space applications. The practical application of MKIDs for photon detection requires a method of efficiently coupling the photon energy to the MKID. To this end we have been studying MKIDs in a strip detector architecture. The second part of this thesis presents our results using strip detectors with tantalum absorbers coupled to aluminum MKIDs.

", } @phdthesis {CaltechTHESIS_https://thesis.library.caltech.edu/id/eprint/559, title ="The Sulfur, Argon, and Calcium Isotopic Composition of the Galactic Cosmic Ray Source", author = "Ogliore, Ryan Christopher", month = "January", year = "2007", doi = "10.7907/ZKDE-TE09", url = "https://resolver.caltech.edu/CaltechETD:etd-02082007-175902", revision_no = "14", abstract = "Galactic cosmic ray measurements of the sulfur, argon, and calcium isotopes made by the Cosmic Ray Isotope Spectrometer on the NASA Advanced Composition Explorer are reported over the energy range from 100 to 400 MeV/nucleon. The propagation of cosmic rays through the Galaxy and heliosphere is modeled with observational constraints imposed by measurements. Source abundance ratios of the sulfur, argon, and calcium isotopes are deduced from this model. Cosmic rays are thought to originate in the cores of superbubbles which contain stellar ejecta mixed with the surrounding interstellar medium. The composition of the superbubble core should reflect the composition of the cosmic rays at their source. Based on the derived isotopic source ratios of sulfur, argon, and calcium, the superbubble material at the cosmic ray source is constrained to be 18%+26%-14% supernova and wind ejecta, with the remainder interstellar medium material. This mix of metal-rich ejecta and interstellar medium in the superbubble core corresponds to a cosmic ray source metallicity of 2.7+3.9-2.1 times solar metallicity.", } @phdthesis {CaltechTHESIS_https://thesis.library.caltech.edu/id/eprint/3760, title ="Gamma-Ray Burst Afterglows: Constraining Physical Parameters and Fireball Model Assumptions", author = "Yost, Sarah Anne", month = "January", year = "2004", doi = "10.7907/FHCW-2P04", url = "https://resolver.caltech.edu/CaltechETD:etd-09252003-152937", revision_no = "12", abstract = "

Gamma-ray bursts (GRBs) are the most luminous events in the high-energy sky. Occurring at cosmological distances, they are cataclysmic events presumed to be associated with the endpoints of massive stars' lives. They are produced by relativistic ejecta. As the ejecta encounters the surrounding medium it slows and emits lower-energy emission known as the GRB afterglow. The evolution of the shock in this phase depends upon the medium encountered as well as on such basic parameters of the event as energy and collimation. Afterglow studies can shed light upon the physics of relativistic shocks and the GRB environment(s), providing indirect clues to the progenitors. These parameters can be determined by fitting afterglow data sets to a model of the event, here, the fireball model. If the model assumptions are correct, the parameters providing a good fit will correspond to those of the event.

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We develop a fireball model starting from its analytic, asymptotic behaviour parameterized by its fundamental parameters (energy, collimation, density, and microphysics). We find good fits to four of the best-sampled broadband afterglow data sets, with simple assumptions concerning the unknown microphysics and circumburst density profile. We present the resulting fit parameters, showing reasonable energies, densities similar to those of diffuse clouds, and a large spread in such microphysical parameters as the fraction of shock energy used to generate magnetic fields.

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We also present results where the model fit showed degeneracies and other data sets that are not well-fit by this model. Motivated to determine the model's inherent uncertainty from the adoption of physical assumption, we consider some changes to these. We present our results: that a range of magnetic energy fraction variation with shock strength is permissible, and that afterglow fits are not sensitive to steeply rising circumburst power law density profiles. We demonstrate that the fitted parameters change when the assumptions are changed; this may be by a small fraction, or up to an order of magnitude.

", } @phdthesis {CaltechTHESIS_https://thesis.library.caltech.edu/id/eprint/2567, title ="Hard X-Ray Observations of the Extragalactic Sky: The High Energy Focusing Telescope and the Serendipitous Extragalactic X-ray Source Identification Survey", author = "Mao, Peter Hsih-Jen", month = "January", year = "2002", doi = "10.7907/R5E3-RQ86", url = "https://resolver.caltech.edu/CaltechETD:etd-06132002-131531", revision_no = "13", abstract = "

Extending the energy range of high sensitivity astronomical x-ray observations to the hard x-ray band (10-100 keV) is important for the study of nonthermal emission mechanisms and heavily obscured sources. This thesis, in two parts, describes the development of the High Energy Focusing Telescope (HEFT), a focusing telescope for the hard x-ray band, and the Serendipitous Extragalactic X-ray Source Identification (SEXSI) survey, a degree-scale x-ray/optical survey of sources detected in the Chandra hard band (2-7 keV).

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HEFT is a balloon-borne x-ray telescope that is expected to have its first flight in the fall of 2003. The telescope will be among the first to focus x-rays at energies greater than 20 keV. HEFT's mirrors use graded multilayers -- thin film coatings (~1 µm) that enhance high energy reflectance via constructive interference. In the first half of the thesis, I describe the optimization algorithm that I developed for x-ray optics and how I applied this algorithm to the design of the HEFT optics. In addition, I present x-ray measurements that verify the HEFT multilayer coating designs at energies where the telescope will operate.

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The SEXSI survey complements Chandra deep-field surveys by covering a much larger area of the sky, but to a shallower x-ray flux limit. For the SEXSI survey, we use public data from the Chandra archive to compile a catalog of extragalactic sources detected in the 2-7 keV band. We identify the optical counterparts to the x-ray sources and obtain their optical spectra (400-1000 nm). Presently SEXSI includes 30 Chandra fields, covering roughly 2 square degrees and yielding over 1200 x-ray sources to a flux limit of 10⁻¹⁵-10⁻¹³ erg cm⁻² s⁻¹. In the second part of the thesis, I present results from 10 fields for which we have substantial spectroscopic coverage.

", } @phdthesis {CaltechTHESIS_https://thesis.library.caltech.edu/id/eprint/3272, title ="Continuous Long Term Observations of Accreting Pulsars", author = "Koh, Danny Towsian", month = "January", year = "1998", doi = "10.7907/ch1h-0w87", url = "https://resolver.caltech.edu/CaltechETD:etd-08292008-082313", revision_no = "17", abstract = "

The all-sky monitoring ability of the BATSE instrument on the GRO has enabled the first uniform, continuous and long term observations of the torque and fluxes of many known accreting pulsars. In the first part of this thesis, I describe the capabilities of BATSE for detecting hard X-rays, the techniques developed to perform timing and flux measurements, and the steps involved in reducing the raw data sets to a standardized database of data products for each accreting pulsar monitored by BATSE.

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In the second part of this thesis, I describe several studies performed with this database. The first is a broad overview of all the accreting pulsars monitored by BATSE. For each source, I display the frequency and pulsed flux histories from ≈ five years of BATSE observations, and provide a brief summary of the BATSE findings.

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I then focus on the wind-fed accreting pulsar GX 301-2. The most striking features in the pulsar frequency history are two steady and rapid spin-up episodes, with v [over-dot] ≈ (3-5) x 10⁻¹² Hz s⁻¹, each lasting for about 30 days. They probably represent the formation of transient accretion disks in this wind-fed pulsar. Except for these spin-up episodes, there are virtually no net changes in the neutron star spin frequency on long time scales. We suggest that the long-term spin-up trend observed since 1984 (v [over-dot] ≈ 2 x 10⁻¹³ Hz s⁻¹) may be due entirely to brief (≈ 20 d) spin-up episodes similar to those we have discovered.

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Next, I highlight the most significant trends revealed by the BATSE observations and discuss their implications for our current understanding of the spin-evolution and torque-luminosity relations in accreting pulsars. Alternating episodes of steady spin-up and spin-down were found to be a surprisingly common characteristic of many persistent sources. Pulsed flux and accretion torque are strongly correlated in outbursts of transient accreting pulsars, but uncorrelated, or even anticorrelated, in persistent sources. I describe the various competing models that currently exist and critically assess each model within the context of the BATSE observations.

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Finally, I describe how BATSE observations of the recurrence rate of transient systems can be used to infer the galactic population of high-mass Be-transient systems.

", } @phdthesis {CaltechTHESIS_https://thesis.library.caltech.edu/id/eprint/4192, title ="Measurements of the isotopic composition of solar energetic particles with the MAST instrument aboard the SAMPEX spacecraft", author = "Williams, Daniel LeRoy", month = "January", year = "1998", doi = "10.7907/P5H0-EB69", url = "https://resolver.caltech.edu/CaltechETD:etd-10202005-144209", revision_no = "10", abstract = "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nWe report measurements of the isotopic composition of solar energetic particles (SEP) as observed in two gradual solar energetic particle events which began on 30 October and 2 November of 1992. These measurements were taken by the Mass Spectrometer Telescope (MAST), a charged-particle telescope on board the SAMPEX satellite in a polar, low-Earth orbit. The elements measured are carbon, nitrogen, oxygen, neon, magnesium and silicon, which are observed over an energy interval of ~15 to 70 MeV per nucleon. These are the first isotopic measurements of silicon, and only the second measurements of the other elements in individual SEP events.\r\n\r\nSAMPEX has access to interplanetary fluxes of energetic particles during the high latitude portions of its orbit. This access depends on the rigidity of the particles and therefore on their charge to mass ratio. Fortunately, the polar orbit of SAMPEX allows measurement of SEP charge states using the geomagnetic filtering technique, and a previous study reported measurements of the ionic charge states in the same two SEP events studied in this work. This was the first time that charge states of SEPs at these high energies have been measured, and it allows rigidity-dependent biases in the elemental and isotopic composition to be investigated in the present study. This is particularly important since acceleration/transport processes depend on rigidity, and because the observed composition is also affected by the Earth's magnetic field.\r\n\r\nThe MAST instrument measures both elemental and isotopic composition using the [...] technique, which resulted in a typical mass resolution of ~0.24 amu. It was found that during times of high counting rates, the measured mass distributions exhibited high-mass tails. Through computer simulation of the instrument and its particle environment, it was found that these tails are caused by [...] MeV protons which hit the instrument hodoscope in coincidence with the heavy ions of interest. By choosing only data which occurred during periods of low to moderate counting rates, the probability of such coincidences could be reduced, and these tails could be effectively removed. The elemental composition was used to find and correct for the bias introduced by these restrictions.\r\n\r\nThe energy spectra of the elements in each SEP event could be fit with exponentials in energy per nucleon, with a single, characteristic e-folding energy for each event. Such well-behaved spectra allowed elemental abundances to be calculated. These abundances exhibited a fractionation due to acceleration and propagation of the SEP source material, which could be ordered as a function of the charge to mass ratio of each element. The SEP source abundances exhibited a depletion of elements with a high first-ionization potential as compared to the composition of the solar photosphere, a phenomena which has been well-observed in SEP composition for many years. The depletion factor varied by a factor of ~2 between the two SEP events.\r\n\r\nThe isotopic ratios of [...], [...], [...], [...] and [...] were found to be in generally good agreement with the solar system values of isotopic composition as tabulated by Anders and Grevesse (1989) based on terrestrial and meteoritic measurements. Earlier measurements of SEP isotopes found a [...] ratio which was ~80% higher than the [...] ratio measured in the solar wind, but close to the neon-A component found in meteorites, which had been adopted as the solar system standard by Cameron (1982). Our measurements of [...], however, are consistent with the [...] ratio found in the solar wind. This leads us to conclude that the underlying isotopic composition of the SEP source material is closer to that of the solar wind.\r\n\r\nThere are only two cases in which the measured isotope ratios differ significantly from the Anders and Grevesse (1989) values, the [...] and the [...] ratio in the second SEP event, both of which are a factor of ~2 higher than the solar system value. There is no apparent pattern to these overabundances.\r\n\r\nWhile the isotopic ratios measured in the two SEP events generally support the assumption that SEPs and solar wind come from the same reservoir of material, and that SEP composition is consistent with the standard solar system abundances compiled by Anders and Grevesse (1989), SEP isotope measurements are still at an exploratory stage. The limited number of observations prevent us from making definite conclusions about the nature of event-to-event variations in SEP isotopic composition, or about possible differences between the coronal and solar system abundances.\r\n", } @phdthesis {CaltechTHESIS_https://thesis.library.caltech.edu/id/eprint/3313, title ="Hard X-Ray Detection and Timing of Accretion-Powered Pulsars with BATSE", author = "Chakrabarty, Deepto", month = "January", year = "1996", url = "https://resolver.caltech.edu/CaltechETD:etd-09022008-132322", revision_no = "21", abstract = "

The BATSE all-sky monitor on the Compton Gamma Ray Observatory is a superb tool for the study of accretion-powered pulsars. In the first part of this thesis, I describe its capabilities for hard X-ray observations above 20 keV, present techniques for timing analysis of the BATSE data, and discuss general statistical issues for the detection of pulsed periodic signals in both the time and frequency domains. BATSE's 1-day pulsed sensitivity in the 20-60 keV range is ≈ 15 mCrab for pulse periods 2 s ≲ Ppulse ≲ 400 s, covering most of the known accreting pulsars. Its sensitivity degrades substantially outside of this range.

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In the second part of this thesis, I present the results of several science investigations applying these techniques. Half the 42 known accreting pulsars have been detected with BATSE and are monitored whenever they are active. Except for a few which lie outside of BATSE's sensitivity range, the rest are all transient sources which may eventually be detected in outburst. The detected systems include four new transients discovered by BATSE, one of which is discussed in detail. A new technique used to localize this source, GRO J1948+32, is described.

\r\n\r\n

Observations of the 38-s pulsar OAO 1657-415 discovered that it is in a 10.4-d eccentric orbit and undergoes regular X-ray eclipses by its massive companion, making it only the seventh known eclipsing X-ray pulsar. Constraints placed by the pulsar mass function and the eclipse duration indicate that the undetected binary companion must be an OB supergiant. If the companion can be identified and its orbital velocity measured, the neutron star mass can be determined.

\r\n\r\n

The 7.7-s pulsar 4U 1626-67 was found to be in an extended spin-down state, ending over a decade of rapid, steady spin-up. It is only the second steady-state disk accreter known to have undergone a torque reversal. The other, the 2-min pulsar GX 1+4, underwent two torque reversals during our observations and is detected up to 160 keV. During spin-down, we find that pulsed flux and torque in GX 1+4 are anticorrelated, the opposite of what is predicted by the usual theories of magnetic accretion torques.

\r\n", } @phdthesis {CaltechTHESIS_https://thesis.library.caltech.edu/id/eprint/4171, title ="High-sensitivity searches for radio pulsars", author = "Ray, Paul Shelton", month = "January", year = "1995", doi = "10.7907/6m41-p432", url = "https://resolver.caltech.edu/CaltechETD:etd-10182007-151414", revision_no = "10", abstract = "Radio pulsars are rapidly spinning, highly magnetized neutron stars which emit beams of radio waves and are observed to pulse when the beam crosses the Earth. They represent the end-point in the evolution of massive stars, and are excellent laboratories for the study of the bulk properties of matter at nuclear densities and beyond. Millisecond pulsars are old pulsars reborn through accretion of matter from a companion star, spinning so fast that the surface velocities approach the speed of light.\r\n\r\nWe describe several high-sensitivity searches for radio pulsars, both for very recently born pulsars in supernova remnants and for ancient millisecond pulsars born early in the history of the Galaxy.\r\n\r\nWe have conducted a survey of 18 supernova remnants for young pulsars using the 305-m radio telescope in Arecibo, Puerto Rico at 430 MHz and 1400 MHz. No pulsars were discovered in this survey which was sensitive to pulsars as faint as 0.2 mJy. The selection effects making pulsars difficult to find in supernova remnants, including high background temperatures of the remnants and high birth velocities of pulsars, are discussed. We conclude that deeper and more extensive surveys are required to constrain the pulsar population in supernova remnants.\r\n\r\nWe have also performed several very large area surveys with excellent sensitivity to pulsars as fast as 1 millisecond, also employing the Arecibo 305-m dish. These surveys will help place limits on the population of low-luminosity pulsars in the Galaxy. A total of 12 non-recycled pulsars were discovered with periods ranging from 96 ms to 2.06 seconds.\r\n\r\nThe primary motivation for these surveys was the discovery of new millisecond pulsars. One 5.9 ms pulsar was discovered and initial timing observations show that it is in a binary system with orbital period 56.2 d and semi-major axis 20.1 lt-s. The implied companion mass is at least 0.2 solar masses. This pulsar, as well as a number of others discovered in recent surveys, are providing excellent laboratories for studies of the formation and evolution of millisecond pulsars, as well as measurements of general relativistic parameters, constraints on the cosmological background of gravitational waves, a pulsar based time standard and dynamical-optical frame ties.\r\n", } @phdthesis {CaltechTHESIS_https://thesis.library.caltech.edu/id/eprint/7666, title ="Multiwavelength observations of the black hole candidate 1E 1740.7-2942", author = "Heindl, William Adams", month = "January", year = "1994", doi = "10.7907/485k-vs32", url = "https://resolver.caltech.edu/CaltechTHESIS:05072013-085136174", revision_no = "16", abstract = "

Observations of the Galactic center region black hole candidate\r\n1E 1740.7-2942 have been carried out using the Caltech Gamma-Ray Imaging Payload\r\n(GRIP), the Röntgensatellit (ROSAT) and the Very Large Array (VLA). These\r\nmultiwavelength observations have helped to establish the association between a\r\nbright emitter of hard X-rays and soft γ-rays, the compact core of a double radio jet\r\nsource, and the X-ray source, 1E 1740.7-2942. They have also provided information\r\non the X-ray and hard X-ray spectrum.

\r\n\r\n

The Galactic center region was observed by GRIP during balloon flights from\r\nAlice Springs, NT, Australia on 1988 April 12 and 1989 April 3. These observations\r\nrevealed that 1E 1740.7-2942 was the strongest source of hard X-rays within ~10°\r\nof the Galactic center. The source spectrum from each flight is well fit by a single\r\npower law in the energy range 35-200 keV. The best-fit photon indices and 100 keV\r\nnormalizations are: γ = (2.05 ± 0.15) and K_(100) = (8.5 ± 0.5) x 10^(-5) cm^(-2) s^(-1) keV^(-1)\r\nand γ = (2.2 ± 0.3) and K_(100) = (7.0 ± 0.7) x 10^(-5) cm^(-2) s^(-1) keV^(-1) for the 1988 and\r\n1989 observations respectively. No flux above 200 keV was detected during either\r\nobservation. These values are consistent with a constant spectrum and indicate that\r\n1E 1740.7-2942 was in its normal hard X-ray emission state. A search on one hour\r\ntime scales showed no evidence for variability.

\r\n\r\n

The ROSAT HRI observed 1E 1740.7-2942 during the period 1991 March\r\n20-24. An improved source location has been derived from this observation. The\r\nbest fit coordinates (J2000) are: Right Ascension = 17^h43^m54^s.9, Declination =\r\n -29°44'45\".3, with a 90% confidence error circle of radius 8\".5. The PSPC observation\r\nwas split between periods from 1992 September 28- October 4 and 1993 March\r\n23-28. A thermal bremsstrahlung model fit to the data yields a column density of\r\nN_H = 1.12^(+1.51)_(0.18) x cm^(-2) , consistent with earlier X- ray measurements.

\r\n\r\n

We observed the region of the Einstein IPC error circle for 1E 1740.7-2942\r\nwith the VLA at 1.5 and 4.9 GHz on 1989 March 2. The 4.9 GHz observation revealed\r\ntwo sources. Source 'A', which is the core of a double aligned radio jet source (Mirabel\r\net al. 1992), lies within our ROSAT error circle, further strengthening its identification\r\nwith 1E 1740.7-2942.

", } @phdthesis {CaltechTHESIS_https://thesis.library.caltech.edu/id/eprint/3261, title ="A study of recycled pulsars in globular clusters", author = "Anderson, Stuart Bruce", month = "January", year = "1993", doi = "10.7907/mb1s-an45", url = "https://resolver.caltech.edu/CaltechETD:etd-08282008-094151", revision_no = "10", abstract = "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nWe have performed a search for radio pulsars in 11 globular clusters visible from the 305 m Arecibo radio telescope. We have found 11 pulsars in the four clusters: M53, M13, NGC6760, and M15; each containing one binary pulsar: 1310+18A, 1639+36B, 1908+00A, and 2127+11C, respectively; as well as a total of seven isolated pulsars: 1639+36A, 2127+11B, 2127+11D, 2127+11E, 2127+11F[superscript 1], 2127+11G, 2127+11H[superscript 1].\r\n\r\nIn this thesis we present the various search strategies employed and discuss the results of pulse time of arrival analysis for the seven isolated pulsars and the binary pulsar 2127+11C, as well as initial Doppler orbits for the binary pulsars 1310+18A and 1639+36B. The remaining pulsar 1908+00A is the subject of current research.\r\n\r\nThe binary pulsar 2127+11C has been found to be in an 8 hour orbit about another degenerate star. The detection of a relativistic precession of the orbit and time dilation effects constrain the pulsar and companion star masses to be [massive pulsar] = 1.34 ± .23 [...] and [mass of companion] = 1.37 ± .23 [...].\r\n\r\nStudy of the radial positions and anomalous spin period derivatives of the seven isolated pulsars in M15 has revealed otherwise unobtainable information on the structure of this post core collapsed globular cluster. From the observed spatial pulsar distribution of [...] it is inferred that the mass of isolated pulsars in M15 is approximately 1.4[...]. From the negative period derivative of PSR 2127+11A,D, it is inferred that the central mass-to-light ratio is > 1.9 [...] /[...] and the central mass density is > 2.7 x 10[superscript 6] [...]pc[superscript -3].\r\n\r\nFrom the observed isolated pulsar luminosity function in M15, [...] and an assumed minimum luminosity of 1.0 mJy kpc[superscript 2] it is inferred that the current population of observable isolated pulsars in M15 is [...](100).\r\n\r\nThe observed dispersion measures towards the eight pulsars in M15 and their relative spatial positions indicate that the spectrum of inhomogeneities in the interstellar medium observed at smaller length scales, extends up to 10[superscript 17] cm.\r\n", } @phdthesis {CaltechTHESIS_https://thesis.library.caltech.edu/id/eprint/7329, title ="Energetic Protons in the Magnetosphere of Neptune", author = "Looper, Mark Dixon", month = "January", year = "1993", url = "https://resolver.caltech.edu/CaltechTHESIS:12122012-085110897", revision_no = "18", abstract = "The Cosmic Ray Subsystem aboard Voyager 2 measured large fluxes of trapped energetic\r\nprotons and electrons in the inner magnetosphere of Neptune during the 1989 flyby; the\r\nprotons above 1.9 MeV observed by the Low Energy Telescopes are analyzed in this thesis.\r\nProton events are extracted from pulse-height distributions dominated by low-energy electron\r\npileup noise, and fluxes are calculated with corrections for discriminator deadtime. Theoretical\r\nmodels for satellite absorption of charged particles are adapted to the large gyroradii of\r\nenergetic protons, and model magnetospheres are constructed that involve diffusion of particles\r\nin the presence of this absorption; parameters of these model magnetospheres are\r\nadjusted to reproduce the observations. The inward-diffusing proton flux is limited by\r\nabsorption due to the moon 1989N1 (Proteus), with absorption at high magnetic latitudes\r\n(whence high Ls) proving to be most important. The proton radial diffusion coefficient is an\r\norder of magnitude less than that inferred elsewhere for the electrons; this prevents protons\r\nfrom diffusing inward past 1989N1 before they are absorbed, and in fact the proton flux\r\nreturns to background levels within a limit well outside the minimum L-shell of 1989N1,\r\nwhile electrons can diffuse past this satellite so that their flux recovers before they are\r\nabsorbed by the other moons and rings closer to the planet. The rate of proton radial\r\ndiffusion, in comparison with that for electrons, is consistent with the diffusion being driven\r\nby electric fields from wind fluctuations in the ionosphere of Neptune. Radial diffusion\r\nalone, however, produces too much pitch-angle anisotropy as particles with mild anisotropy\r\nin the outer magnetosphere are transported inward, and pitch-angle diffusion must be invoked\r\nto reduce the excess anisotropy and reproduce the observations. The pitch-angle distributions\r\nat different Ls are consistent with the diffusion coefficient for this process being comparable\r\nin magnitude to that for radial diffusion inside L of about 6.8, though still much less than the\r\nstrong-diffusion limit, and negligible outside that L.\r\n", } @phdthesis {CaltechTHESIS_https://thesis.library.caltech.edu/id/eprint/6645, title ="Gamma-ray imaging observations of Supernova 1987A", author = "Palmer, David M.", month = "January", year = "1992", doi = "10.7907/ctqh-qa26", url = "https://resolver.caltech.edu/CaltechTHESIS:09012011-153518265", revision_no = "20", abstract = "The Caltech imaging γ-ray telescope has made four balloon flights from Alice Springs, Australia, to observe the hard X-ray and γ-ray emission from Supernova 1987 A as it evolved between 1987 May and 1989 April. We have detected γ-rays with the time behavior and spectral signature expected from freshly-synthesized radioisotopes embedded in a cloud of ejecta. In particular, we detect the 847 and 1238 keY γ-ray lines produced by the decay of ^(56)Co, and the continuum spectrum expected from Compton scattering of these γ-trays. The results of these observations are compared with other measurements and with theoretical models of supernovae, and it is found that our results are consistent with core-collapse models for SN 1987A in which the centrally-produced radioisotopes have propagated outwards to mix with the ejecta.\r\n", } @phdthesis {CaltechTHESIS_https://thesis.library.caltech.edu/id/eprint/6650, title ="Isotopic composition of galactic cosmic ray boron, carbon, nitrogen, and oxygen", author = "Gibner, Peyton Stinson", month = "January", year = "1992", doi = "10.7907/m2hn-q437", url = "https://resolver.caltech.edu/CaltechTHESIS:09062011-105435728", revision_no = "14", abstract = "

Relative abundances of the isotopes of galactic cosmic ray B, C, N, and O nuclei have been measured using the balloon-borne High Energy Isotope Spectrometer Telescope (HEIST). Analysis of data collected during the 1988 HEIST flight from Prince Albert, Saskatchewan, has resulted in mass histograms containing ~890 boron, ~3100 carbon, ~910 nitrogen, and ~3300 oxygen nuclei. Masses were derived using both the Cerenkov-Energy and ΔE-E' techniques, achieving a resulting rms mass resolution of ~0.26 amu. These isotopic composition measurements correspond to energy intervals at the top of the atmosphere of ~400-650 MeV/nucleon for boron, 430-670 MeV/nucleon for carbon, 440-680 MeV/nucleon for nitrogen, and 450-780 MeV/nucleon for oxygen, higher than previous direct isotope measurements for these elements.

\r\n\r\n

The abundance ratios of carbon, nitrogen, and oxygen at the top of the atmosphere have been interpreted using an interstellar propagation model that includes improved fragmentation cross sections. Because cosmic ray boron is used as a \"secondary tracer,\" the calculated isotope ratios of interest are insensitive to the value chosen for the solar modulation parameter, ø. The resulting abundance ratios for cosmic ray source material include ^(14)N/O = 0.042 ± 0.014 and ^(15)N/O ≤ 0.040, favoring no ^(15)N at the source. The carbon and oxygen isotopes at the cosmic ray source are ^(13)C/^(12)C = 0.005 ± .011 and ^(18)O/^(16)O = 0.0115 ± .0038, compared to solar system values of ^(13)C/^(12)C = 0.011 and ^(18)O/^(16)O = 0.0020. The derived cosmic ray source abundances show a possible enhancement of ^(18)O/^(16)O over the solar system value and a ^(13)C/^(12)C ratio consistent with solar system material. Taking a weighted average of our result with previous high resolution measurements of oxygen results in ^(18)O/^(16)O = 0.0075 ± 0.0024, an enhancement in ^(18)O of 3.75 times the solar system value.

\r\n\r\n

Current isotope results are compared with models of cosmic ray origin. Both the supermetallicity model and the \"anomalous\" solar system model predict an ^(18)O excess in cosmic rays, however, the \"anomalous\" solar system model also predicts an excess in ^(13)C. The Wolf-Rayet model fits many of the currently observed isotopic excesses in cosmic rays, but the predictions for ^(18)O/^(16)O and the elemental N/O ratio are still in question. We conclude that although further refinements in the Wolf-Rayet model may explain ^(18)O and N/O, none of the presently available models account quantitatively for all of the observed differences in composition between cosmic rays and solar system material.

\r\n", } @phdthesis {CaltechTHESIS_https://thesis.library.caltech.edu/id/eprint/2338, title ="A Balloon Measurement of the Isotopic Composition of Galactic Cosmic Ray Iron", author = "Grove, Jon Eric", month = "January", year = "1989", doi = "10.7907/3cr8-jv31", url = "https://resolver.caltech.edu/CaltechETD:etd-05312007-121425", revision_no = "14", abstract = "

We have measured the isotopic composition of galactic cosmic ray iron in the energy interval ~1550-2200 MeV/nucleon using a balloon-borne mass spectrometer. The instrument was flown from Palestine, Texas, in May 1984 for >35 hours at an atmospheric depth of ~6 g/cm². Masses were derived by the Cerenkov-Energy technique. The Cerenkov counter employed a silica aerogel radiator with index of refraction n = 1.1. Particle energies were measured in a stack of NaI(Tl) scintillators, which also provided particle trajectories. The calibration of the detectors is discussed, along with the algorithms we have used to calculate velocities, energies, and masses. The limitations of aerogels as Cerenkov radiators, particularly the stability of their light yield, are considered. A detailed discussion of the sources of mass uncertainty is presented, including an analytic model of the contribution from fluctuations in the Cerenkov yield from knock-on electrons. The achieved mass resolution is ~0.65 amu, which is consistent with the theoretical estimate. We report an ⁵⁴Fe/⁵⁶Fe abundance ratio of 0.14+0.18-0.11 and an 84% confidence upper limit of ⁵⁸Fe/⁵⁶Fe ≤ 0.07 at the top of the atmosphere. Combining our data with those of previous measurements of the composition of iron at lower energies, and using a model of the galactic propagation, we derive cosmic-ray source abundance ratios of ⁵⁴Fe/⁵⁶Fe = 0.064+0.032-0.027 and ⁵⁸Fe/⁵⁶Fe ≤ 0.062. These values are consistent with the composition of solar-system iron and place restrictions on the conditions under which cosmic-ray iron is synthesized.

", } @phdthesis {CaltechTHESIS_https://thesis.library.caltech.edu/id/eprint/2309, title ="Evidence for Anomalous Cosmic Ray Hydrogen", author = "Christian, Eric Richard", month = "January", year = "1989", doi = "10.7907/AQ34-AJ87", url = "https://resolver.caltech.edu/CaltechETD:etd-05302007-091830", revision_no = "17", abstract = "

The High Energy Telescopes on the Voyager 1 and Voyager 2 spacecraft are used to measure the differential energy spectra of hydrogen and other elements. During the period of minimum solar modulation in 1987, changes in the shape of the hydrogen energy spectra are observed. It is shown that these changes are difficult to explain in the framework of current modulation theory, and are consistent with the emergence of an anomalous cosmic ray (ACR) hydrogen component. ACR hydrogen is predicted by the current theories of anomalous cosmic rays, but this is the first evidence that ACR hydrogen is actually present. Several different estimates of the contribution of ACR hydrogen are used to obtain peak fluxes of 0.33 ± 0.12 particles/m2s sr MeV and 0.67 ± 0.18 particles/m2s sr MeV for Voyager 1 and Voyager 2 respectively during the time period 1987/209-313.

\r\n\r\n

Using a model developed by Cummings and Stone (1987), we relate these fluxes of ACR hydrogen and the peak fluxes of ACR helium to the relative abundance of hydrogen and helium, n(H I)/n(He I), in the neutral gas flowing into the solar system from the local interstellar medium. For two different choices of parameters, we obtain values of 3 ± 1 and 5 ± 3 for n(H I)/n(He I), which should be compared to the cosmic relative abundance of ~ 10. Our values are consistent with previous results obtained from solar ultraviolet backscatter experiments, and support the hypothesis that hydrogen is substantially ionized in the very local interstellar medium.

", } @phdthesis {CaltechTHESIS_https://thesis.library.caltech.edu/id/eprint/6690, title ="The Imaging of Extra-Galactic Low-Energy Gamma-Ray Sources: Prospects, Techniques, and Instrumentation", author = "Finger, Mark Harold", month = "January", year = "1988", doi = "10.7907/yagn-1j29", url = "https://resolver.caltech.edu/CaltechTHESIS:09272011-105853397", revision_no = "27", abstract = "

The handful of extra-galactic low-energy gamma-ray sources so far observed are all active galaxies, which are expected to dominate future detections. The nature of these compact, highly luminous sources is at present not clear; however, they may be powered by massive black holes. Many of these sources may produce their peak luminosity in the 0.5 to 5.0 MeV energy band, and observation in this energy range will be important in revealing the nature of their central power-house.

\r\n\r\n

Improved understanding of the nature of active galaxies will require detailed observations of 10-20 sources, while understanding of their gamma-ray luminosity function and its evolution will require the detection of ~100 sources. From x-ray number counts and the presently available information about active galaxy spectra, we estimate the hard x-ray and low-energy gamma-ray number source-flux relation N(>S) for active galaxies. Instruments capable of detecting ~100 active galaxies at low-energy gamma-ray energies are achievable. These instruments will, however, be observing sources with fluxes some 10⁻³ - 10⁻⁴ times lower than their instrumental background level, and will require careful control of systematic errors.

\r\n\r\n

The angular resolution of an instrument, as well as its sensitivity, can limit the number of sources it can observe. We present an investigation of the angular resolution requirements for future low-energy gamma-ray instruments. We find that the strictest requirements arise not from the need to resolve detectable sources, but from the need to control the level of direction-to-direction fluctuations in the diffuse background level. We conclude that gamma-ray instruments capable of detecting 100 active galaxies must have sub-degree angular resolution.

\r\n\r\n

We propose use of the coded aperture imaging technique as a method of achieving accurate control of systematic errors and fine angular resolution without unduly increasing the time needed to conduct full-sky surveys. This is a technique that employs a partially opaque mask to spatially modulate the source flux incident upon a position-sensitive photon detector. We present an analysis of coded aperture imaging for instruments that employ masks based on hexagonal uniformly redundant arrays. Rotation of such a mask allows complete, position-by-position background subtraction on short time-scales, and removes the periodic ambiguity inherent in uniformly redundant arrays.

\r\n\r\n

An instrument, the Gamma-ray Imaging Payload, has been built that employs these imaging techniques. The primary detector of the instrument is a 41 cm diameter by 5 cm thick NaI(Tl) Anger camera. We describe the design and testing of the instrument in detail. Preliminary results from a balloon flight of the instrument are shown, demonstrating its imaging performance.

", } @phdthesis {CaltechTHESIS_https://thesis.library.caltech.edu/id/eprint/8688, title ="The Abundances of Ultraheavy Elements in the Cosmic Radiation", author = "Newport, Brian John", month = "January", year = "1986", doi = "10.7907/2bfw-r090", url = "https://resolver.caltech.edu/CaltechTHESIS:10092014-143512211", revision_no = "31", abstract = "

Analysis of the data from the Heavy Nuclei Experiment on the HEAO-3 spacecraft has yielded the cosmic ray abundances of odd-even element pairs with atomic number, Z, in the range 33 ≤ Z ≤ 60, and the abundances of broad element groups in the range 62 ≤ Z ≤ 83, relative to iron. These data show that the cosmic ray source composition in this charge range is quite similar to that of the solar system provided an allowance is made for a source fractionation based on first ionization potential. The observations are inconsistent with a source composition which is dominated by either r-process or s-process material, whether or not an allowance is made for first ionization potential. Although the observations do not exclude a source containing the same mixture of r- and s-process material as in the solar system. the data are best fit by a source having an r- to s-process ratio of 1.22+0.25-0.21, relative to the solar system The abundances of secondary elements are consistent with the leaky box model of galactic propagation, implying a pathlength distribution similar to that which explains the abundances of nuclei with Z < 29.

\r\n\r\n

The energy spectra of the even elements in the range 38 ≤ Z ≤ 60 are found to have a deficiency of particles in the range ~1.5 to 3 GeV/amu, compared to iron. This deficiency may result from ionization energy loss in the interstellar medium, and is not predicted by propagation models which ignore such losses. ln addition, the energy spectra of secondary elements are found to be different to those of the primary elements. Such effects are consistent with observations of lighter nuclei, and are in qualitative agreement with galactic propagation models using a rigidity dependent escape length. The energy spectra of secondaries arising from the platinum group are found to be much steeper than those of lower Z. This effect may result from energy dependent fragmentation cross sections.

\r\n\r\n", } @phdthesis {CaltechTHESIS_https://thesis.library.caltech.edu/id/eprint/8692, title ="A Cerenkov-ΔΕ-Cerenkov Detector for High Energy Cosmic Ray Isotopes and an Accelerator Study of ⁴⁰Ar & ⁵⁶Fe Fragmentation", author = "Lau, Koon Hang", month = "January", year = "1985", doi = "10.7907/m2gd-r904", url = "https://resolver.caltech.edu/CaltechTHESIS:10142014-102314134", note = "Thesis title listed in 1985 Commencement Program varies from actual thesis: A Cerenkov-EΔ-Cerenkov Detector for Measuring High Energy Heavy Cosmic Ray Isotope and Studies of Fragmentation of ⁴⁰Ar and ⁵⁶Fe.", revision_no = "25", abstract = "

This thesis has two major parts. The first part of the thesis will describe a high energy cosmic ray detector -- the High Energy Isotope Spectrometer Telescope (HEIST). HEIST is a large area (0.25 m2sr) balloon-borne isotope spectrometer designed to make high-resolution measurements of isotopes in the element range from neon to nickel (10 ≤ Z ≤ 28) at energies of about 2 GeV/nucleon. The instrument consists of a stack of 12 NaI(Tl) scintilla tors, two Cerenkov counters, and two plastic scintillators. Each of the 2-cm thick NaI disks is viewed by six 1.5-inch photomultipliers whose combined outputs measure the energy deposition in that layer. In addition, the six outputs from each disk are compared to determine the position at which incident nuclei traverse each layer to an accuracy of ~2 mm. The Cerenkov counters, which measure particle velocity, are each viewed by twelve 5-inch photomultipliers using light integration boxes.

\r\n\r\n

HEIST-2 determines the mass of individual nuclei by measuring both the change in the Lorentz factor (Δγ) that results from traversing the NaI stack, and the energy loss (ΔΕ) in the stack. Since the total energy of an isotope is given by Ε = γM, the mass M can be determined by M = ΔΕ/Δγ. The instrument is designed to achieve a typical mass resolution of 0.2 amu.

\r\n\r\n

The second part of this thesis presents an experimental measurement of the isotopic composition of the fragments from the breakup of high energy 40Ar and 56Fe nuclei. Cosmic ray composition studies rely heavily on semi-empirical estimates of the cross-sections for the nuclear fragmentation reactions which alter the composition during propagation through the interstellar medium. Experimentally measured yields of isotopes from the fragmentation of 40Ar and 56Fe are compared with calculated yields based on semi-empirical cross-section formulae. There are two sets of measurements. The first set of measurements, made at the Lawrence Berkeley Laboratory Bevalac using a beam of 287 MeV/nucleon 40Ar incident on a CH2 target, achieves excellent mass resolution (σm ≤ 0.2 amu) for isotopes of Mg through K using a Si(Li) detector telescope. The second set of measurements, also made at the Lawrence Berkeley Laboratory Bevalac, using a beam of 583 MeV/nucleon 56Fe incident on a CH2 target, resolved Cr, Mn, and Fe fragments with a typical mass resolution of ~ 0.25 amu, through the use of the Heavy Isotope Spectrometer Telescope (HIST) which was later carried into space on ISEE-3 in 1978. The general agreement between calculation and experiment is good, but some significant differences are reported here.

\r\n", } @phdthesis {CaltechTHESIS_https://thesis.library.caltech.edu/id/eprint/8686, title ="Solar Photospheric and Coronal Abundances from Solar Energetic Particle Measurements", author = "Breneman, Hugh Herbert, III", month = "January", year = "1985", doi = "10.7907/4mma-e885", url = "https://resolver.caltech.edu/CaltechTHESIS:10092014-111031305", revision_no = "23", abstract = "

Observations of solar energetic particles (SEPs) from 22 solar flares in the 1977-1982 time period are reported. The observations were made by the Cosmic Ray Subsystem on board the Voyager 1 and 2 spacecraft. SEP abundances have been obtained for all elements with 3 ≤ Z ≤ 30 except Li, Be, B, F, Sc, V, Co and Cu, for which upper limits have been obtained. Statistically meaningful abundances of several rare elements (e.g., P, Cl, K, Ti, Mn) have been determined for the first time, and the average abundances of the more abundant elements have been determined with improved precision, typically a factor of three better than the best previous determinations.

\r\n\r\n

Previously reported results concerning the dependence of the fractionation of SEPs relative to photosphere on first ionization potential (FIP) have been confirmed and amplified upon with the new data. The monotonic Z-dependence of the variation between flares noted by earlier studies was found to be interpretable as a fractionation, produced by acceleration of the particles from the corona and their propagation through interplanetary space, which is ordered by the ionic charge-to-mass ratio Q/ M of the species making up the SEPs. It was found that Q/M is the primary organizing parameter of acceleration and propagation effects in SEPs, as evidenced by the dependence on Q/M of time, spatial and energy dependence within flares and of the abundance variability from flare to flare.

\r\n\r\n

An unfractionated coronal composition was derived by applying a simple Q/M fractionation correction to the observed average SEP composition, to simultaneously correct for all Q/M-correlated acceleration/propagation fractionation of SEPs. The resulting coronal composition agrees well with current XUV/X-ray spectroscopic measurements of coronal composition but is of much higher precision and is available for a much larger set of elements. Compared to spectroscopic photospheric abundances, the SEP-derived corona appears depleted in C and somewhat enriched in Cr (and possibly Ca and Ti).

\r\n\r\n

An unfractionated photospheric composition was derived by applying a simple FIP fractionation correction to the derived coronal composition, to correct for the FIP-associated fractionation of the corona during its formation from photospheric material. The resulting composition agrees well with the photospheric abundance tabulation of Grevesse (1984) except for an at least ~50% lower abundance of C and a significantly greater abundance of Cr and possibly Ti. The results support the Grevesse photospheric Fe abundance, about 50% higher than meteoritic and earlier solar values. The SEP-derived photospheric composition is not generally of higher precision than the available spectroscopic data, but it relies on fewer physical parameters and is available for some elements (C, N, Ne, Ar) which cannot be measured spectroscopically in the photosphere.

", }