[ { "id": "authors:hbvx1-fms77", "collection": "authors", "collection_id": "hbvx1-fms77", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20211213-518528000", "type": "article", "title": "MOJAVE. XIX. Brightness Temperatures and Intrinsic Properties of Blazar Jets", "author": [ { "family_name": "Homan", "given_name": "D. C.", "orcid": "0000-0002-4431-0890", "clpid": "Homan-Daniel-C" }, { "family_name": "Cohen", "given_name": "M. H.", "orcid": "0000-0002-8033-5972", "clpid": "Cohen-M-H" }, { "family_name": "Hovatta", "given_name": "T.", "orcid": "0000-0002-2024-8199", "clpid": "Hovatta-Talvikki" }, { "family_name": "Kellermann", "given_name": "K. I.", "orcid": "0000-0002-0093-4917", "clpid": "Kellermann-Kenneth-I" }, { "family_name": "Kovalev", "given_name": "Y. Y.", "orcid": "0000-0001-9303-3263", "clpid": "Kovalev-Yuri-Y" }, { "family_name": "Lister", "given_name": "M. L.", "orcid": "0000-0003-1315-3412", "clpid": "Lister-Mattthew-L" }, { "family_name": "Popkov", "given_name": "A. V.", "orcid": "0000-0002-0739-700X", "clpid": "Popkov-Alexander" }, { "family_name": "Pushkarev", "given_name": "A. B.", "orcid": "0000-0002-9702-2307", "clpid": "Pushkarev-Alexander-B" }, { "family_name": "Ros", "given_name": "E.", "orcid": "0000-0001-9503-4892", "clpid": "Ros-Eduardo" }, { "family_name": "Savolainen", "given_name": "T.", "orcid": "0000-0001-6214-1085", "clpid": "Savolainen-Tuomas" } ], "abstract": "We present multiepoch, parsec-scale core brightness temperature observations of 447 active galactic nucleus (AGN) jets from the MOJAVE and 2 cm Survey programs at 15 GHz from 1994 to 2019. The brightness temperature of each jet over time is characterized by its median value and variability. We find that the range of median brightness temperatures for AGN jets in our sample is much larger than the variations within individual jets, consistent with Doppler boosting being the primary difference between the brightness temperatures of jets in their median state. We combine the observed median brightness temperatures with apparent jet speed measurements to find the typical intrinsic Gaussian brightness temperature of 4.1( \u00b1 0.6) \u00d7 10\u00b9\u2070 K, suggesting that jet cores are at or below equipartition between particle and magnetic field energy in their median state. We use this value to derive estimates for the Doppler factor for every source in our sample. For the 309 jets with both apparent speed and brightness temperature data, we estimate their Lorentz factors and viewing angles to the line of sight. Within the BL Lac optical class, we find that high-synchrotron-peaked BL Lacs have smaller Doppler factors, lower Lorentz factors, and larger angles to the line of sight than intermediate and low-synchrotron-peaked BL Lacs. We confirm that AGN jets with larger Doppler factors measured in their parsec-scale radio cores are more likely to be detected in \u03b3 rays, and we find a strong correlation between \u03b3-ray luminosity and Doppler factor for the detected sources.", "doi": "10.3847/1538-4357/ac27af", "issn": "0004-637X", "publisher": "American Astronomical Society", "publication": "Astrophysical Journal", "publication_date": "2021-12-10", "series_number": "1", "volume": "923", "issue": "1", "pages": "Art. No. 67" }, { "id": "authors:jbayp-syz78", "collection": "authors", "collection_id": "jbayp-syz78", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20200421-130913175", "type": "article", "title": "180\u00b0 rotations in the polarization angle for blazars", "author": [ { "family_name": "Cohen", "given_name": "M. H.", "orcid": "0000-0002-8033-5972", "clpid": "Cohen-M-H" }, { "family_name": "Savolainen", "given_name": "T.", "orcid": "0000-0001-6214-1085", "clpid": "Savolainen-T" } ], "abstract": "Rotations of the electric vector position angle (EVPA) in blazars are often close to an integral multiple of 180\u00b0. There are many examples of this in the literature, and we strengthen the evidence by showing that, in the RoboPol monitoring program, n\u03c0 rotations occur more frequently than otherwise expected by chance. We explain this using a model consisting of two polarized emission components: a \"jet\" that is constant in time and a \"burst\" that is variable. The EVPA of the combination is EVPA_(jet) at both the beginning and the end of the burst, so the net rotation across the burst must be n\u03c0. Several examples of this model are analyzed on the Stokes plane, where the winding number for the Stokes vector of the combination gives the value of n. The main conclusion is that the EVPA rotation can be much larger than the physical rotation of the emission region around the axis of the jet, but this requires the EVPAs of the jet and the burst to be nearly orthogonal. Shock-in-jet calculations can provide a physical model for our toy model and in addition they automatically give the required orthogonality. The model is illustrated with data from the literature on OJ 287. We suggest that the large rapid EVPA rotation seen in OJ 287 might be a phase effect and not representative of a physical rotation.", "doi": "10.1051/0004-6361/201936907", "issn": "0004-6361", "publisher": "EDP Sciences", "publication": "Astronomy and Astrophysics", "publication_date": "2020-04", "volume": "636", "pages": "Art. No. A79" }, { "id": "authors:dcmxf-t2w12", "collection": "authors", "collection_id": "dcmxf-t2w12", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20180808-133918006", "type": "article", "title": "Reversals in the Direction of Polarization Rotation in OJ 287", "author": [ { "family_name": "Cohen", "given_name": "M. H.", "orcid": "0000-0002-8033-5972", "clpid": "Cohen-M-H" }, { "family_name": "Aller", "given_name": "H. D.", "clpid": "Aller-H-D" }, { "family_name": "Aller", "given_name": "M. F.", "orcid": "0000-0003-2483-2103", "clpid": "Aller-M-F" }, { "family_name": "Hovatta", "given_name": "T.", "orcid": "0000-0002-2024-8199", "clpid": "Hovatta-T" }, { "family_name": "Kharb", "given_name": "P.", "orcid": "0000-0003-3203-1613", "clpid": "Kharb-P" }, { "family_name": "Kovalev", "given_name": "Y. Y.", "orcid": "0000-0001-9303-3263", "clpid": "Kovalev-Y-Y" }, { "family_name": "Lister", "given_name": "M. L.", "orcid": "0000-0003-1315-3412", "clpid": "Lister-M-L" }, { "family_name": "Meier", "given_name": "D. L.", "clpid": "Meier-D-L" }, { "family_name": "Pushkarev", "given_name": "A. B.", "orcid": "0000-0002-9702-2307", "clpid": "Pushkarev-A-B" }, { "family_name": "Savolainen", "given_name": "T.", "orcid": "0000-0001-6214-1085", "clpid": "Savolainen-T" } ], "abstract": "We have obtained a smooth time series for the electric vector position angle (EVPA) of the blazar OJ 287 at centimeter wavelengths, by making \u00b1n\u03c0 adjustments to archival values from 1974 to 2016. The data display rotation reversals in which the EVPA rotates counterclockwise for ~180\u00b0 and then rotates clockwise by a similar amount. The timescale of the rotations is a few weeks to a year, and the scale for a double rotation, including the reversal, is 1\u20133 yr. We have seen four of these events in 40 yr. A model consisting of two successive outbursts in polarized flux density, with EVPAs counterrotating, superposed on a steady polarized jet, can explain many of the details of the observations. Polarization images support this interpretation. The model can also help to explain similar events seen at optical wavelengths. The outbursts needed for the model can be generated by the supermagnetosonic jet model of Nakamura et al. and Nakamura & Meier, which requires a strong helical magnetic field. This model produces forward and reverse pairs of fast and slow MHD waves, and the plasma inside the two fast/slow pairs rotates around the jet axis, but in opposite directions.", "doi": "10.3847/1538-4357/aacb31", "issn": "1538-4357", "publisher": "American Astronomical Society", "publication": "Astrophysical Journal", "publication_date": "2018-07-20", "series_number": "1", "volume": "862", "issue": "1", "pages": "Art. No. 1" }, { "id": "authors:jnz1c-zat62", "collection": "authors", "collection_id": "jnz1c-zat62", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20180430-101113523", "type": "article", "title": "OJ 287 as a Rotating Helix", "author": [ { "family_name": "Cohen", "given_name": "Marshall H.", "orcid": "0000-0002-8033-5972", "clpid": "Cohen-M-H" } ], "abstract": "We present preliminary data from high-cadence 15-GHz VLBA images of OJ 287 from 1995 to 2015. The ridgelines suggest that the jet is rotating, perhaps with a period of\u223c30 years. The EVPA of the core rotated by 240\u00b0 in 2001\u20132002 and decreased slowly after that. The inner jet apparently moved to a new direction after the rotation,as shown by the emergence of a new component at a new PA at 43 GHz, in 2004. This was presaged by a strong rise in the flux density of the core, and then its sudden fall as the new component was identified. The equivalent sequence of events took place about 5 years later at 15 GHz, but in addition the core EVPA had a step in 2006 and moved to be aligned with the new 43-GHz component. The 15-GHz core became optically thin in 2006, but the angular resolution was insufficient to separate the new component from the core until 2010.", "doi": "10.3390/galaxies5010012", "issn": "2075-4434", "publisher": "MDPI", "publication": "Galaxies", "publication_date": "2017-02-08", "series_number": "1", "volume": "5", "issue": "1", "pages": "Art. No. 12" }, { "id": "authors:8vze6-9xp41", "collection": "authors", "collection_id": "8vze6-9xp41", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20150112-144746772", "type": "article", "title": "Studies of the Jet in BL Lacertae. II. Superluminal Alfv\u00e9n Waves", "author": [ { "family_name": "Cohen", "given_name": "M. H.", "orcid": "0000-0002-8033-5972", "clpid": "Cohen-M-H" }, { "family_name": "Meier", "given_name": "D. L.", "clpid": "Meier-D-L" }, { "family_name": "Arshakian", "given_name": "T. G.", "clpid": "Arshakian-T-G" }, { "family_name": "Clausen-Brown", "given_name": "E.", "clpid": "Clausen-Brown-E" }, { "family_name": "Homan", "given_name": "D. C.", "orcid": "0000-0002-4431-0890", "clpid": "Homan-D-C" }, { "family_name": "Hovatta", "given_name": "T.", "orcid": "0000-0002-2024-8199", "clpid": "Hovatta-T" }, { "family_name": "Kovalev", "given_name": "Y. Y.", "orcid": "0000-0001-9303-3263", "clpid": "Kovalev-Y-Y" }, { "family_name": "Lister", "given_name": "M. L.", "orcid": "0000-0003-1315-3412", "clpid": "Lister-M-L" }, { "family_name": "Pushkarev", "given_name": "A. B.", "orcid": "0000-0002-9702-2307", "clpid": "Pushkarev-A-B" }, { "family_name": "Richards", "given_name": "J. L.", "clpid": "Richards-J-L" }, { "family_name": "Savolainen", "given_name": "T.", "orcid": "0000-0001-6214-1085", "clpid": "Savolainen-T" } ], "abstract": "We study the kinematics of ridge lines on the pc-scale jet of the active galactic nucleus BL Lac. We show that the ridge lines display transverse patterns that move superluminally downstream, and that the moving patterns are analogous to waves on a whip. Their apparent speeds \u03b2_(app) (units of c) range from 3.9 to 13.5, corresponding to \u03b2gal/wave=0.981\u22120.998 in the galaxy frame. We show that the magnetic field in the jet is well-ordered with a strong transverse component, and assume that it is helical and that the transverse patterns are Alfv\u00e9n waves propagating downstream on the longitudinal component of the magnetic field. The wave-induced transverse speed of the jet is non-relativistic (\u03b2gal/tr\u22720.09). In 2010 the wave activity subsided and the jet then displayed a mild wiggle that had a complex oscillatory behaviour. The Alfv\u00e9n waves appear to be excited by changes in the position angle of the recollimation shock, in analogy to exciting a wave on a whip by shaking the handle. A simple model of the system with plasma sound speed \u03b2_s=0.3 and apparent speed of a slow MHD wave \u03b2_(app,S)=4 yields Lorentz factor of the beam \u0393_(beam)\u223c4.5, pitch angle of the helix (in the beam frame) \u03b1\u223c67^\u2218, Alfv\u00e9n speed \u03b2_A\u223c0.64, and magnetosonic Mach number M_(ms)\u223c4.7. This describes a plasma in which the magnetic field is dominant and in a rather tight helix, and Alfv\u00e9n waves are responsible for the moving transverse patterns.", "doi": "10.1088/0004-637X/803/1/3", "issn": "0004-637X", "publisher": "American Astronomical Society", "publication": "Astrophysical Journal", "publication_date": "2015-04-10", "series_number": "1", "volume": "803", "issue": "1", "pages": "Art. No. 3" }, { "id": "authors:23t9d-fc182", "collection": "authors", "collection_id": "23t9d-fc182", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20140619-130354021", "type": "article", "title": "Studies of the Jet in Bl Lacertae. I. Recollimation Shock and Moving Emission Features", "author": [ { "family_name": "Cohen", "given_name": "M. H.", "orcid": "0000-0002-8033-5972", "clpid": "Cohen-M-H" }, { "family_name": "Meier", "given_name": "D. L.", "clpid": "Meier-D-L" }, { "family_name": "Arshakian", "given_name": "T. G.", "clpid": "Arshakian-T-G" }, { "family_name": "Homan", "given_name": "D. C.", "orcid": "0000-0002-4431-0890", "clpid": "Homan-D-C" }, { "family_name": "Hovatta", "given_name": "T.", "orcid": "0000-0002-2024-8199", "clpid": "Hovatta-T" }, { "family_name": "Kovalev", "given_name": "Y. Y.", "orcid": "0000-0001-9303-3263", "clpid": "Kovalev-Y-Y" }, { "family_name": "Lister", "given_name": "M. L.", "orcid": "0000-0003-1315-3412", "clpid": "Lister-M-L" }, { "family_name": "Pushkarev", "given_name": "A. B.", "orcid": "0000-0002-9702-2307", "clpid": "Pushkarev-A-B" }, { "family_name": "Richards", "given_name": "J. L.", "clpid": "Richards-J-L" }, { "family_name": "Savolainen", "given_name": "T.", "orcid": "0000-0001-6214-1085", "clpid": "Savolainen-T" } ], "abstract": "Parsec-scale VLBA images of BL Lac at 15 GHz show that the jet contains a permanent quasi-stationary emission feature 0.26 mas (0.34 pc projected) from the core, along with numerous moving features. In projection, the tracks of the moving features cluster around an axis at a position angle of \u2013166\u00ba.6 that connects the core with the standing feature. The moving features appear to emanate from the standing feature in a manner strikingly similar to the results of numerical two-dimensional relativistic magneto-hydrodynamic (RMHD) simulations in which moving shocks are generated at a recollimation shock (RCS). Because of this, and the close analogy to the jet feature HST-1 in M87, we identify the standing feature in BL Lac as an RCS. We assume that the magnetic field dominates the dynamics in the jet, and that the field is predominantly toroidal. From this we suggest that the moving features are compressions established by slow and fast mode magneto-acoustic MHD waves. We illustrate the situation with a simple model in which the slowest moving feature is a slow-mode wave, and the fastest feature is a fast-mode wave. In the model, the beam has Lorentz factor \u0393_(beam)^(gal) \u2248 3.5 in the frame of the host galaxy and the fast mode wave has Lorentz factor \u0393_(Fwave)^(beam) \u2248 1.6 in the frame of the beam. This gives a maximum apparent speed for the moving features, \u03b2_(app) = v_(app)/c = 10. In this model the Lorentz factor of the pattern in the galaxy frame is approximately three times larger than that of the beam itself.", "doi": "10.1088/0004-637X/787/2/151", "issn": "0004-637X", "publisher": "American Astronomical Society", "publication": "Astrophysical Journal", "publication_date": "2014-06-01", "series_number": "2", "volume": "787", "issue": "2", "pages": "Art. No. 151" }, { "id": "authors:4jx8v-myj67", "collection": "authors", "collection_id": "4jx8v-myj67", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20091211-141725203", "type": "article", "title": "MOJAVE: Monitoring of Jets in Active Galactic Nuclei with VLBA Experiments. VI. Kinematics Analysis of a Complete Sample of Blazar Jets", "author": [ { "family_name": "Lister", "given_name": "M. L.", "orcid": "0000-0003-1315-3412", "clpid": "Lister-M-L" }, { "family_name": "Cohen", "given_name": "M. H.", "orcid": "0000-0002-8033-5972", "clpid": "Cohen-M-H" }, { "family_name": "Homan", "given_name": "D. C.", "orcid": "0000-0002-4431-0890", "clpid": "Homan-D-C" }, { "family_name": "Kadler", "given_name": "M.", "clpid": "Kadler-M" }, { "family_name": "Kellermann", "given_name": "K. I.", "orcid": "0000-0002-0093-4917", "clpid": "Kellermann-K-I" }, { "family_name": "Kovalev", "given_name": "Y. Y.", "orcid": "0000-0001-9303-3263", "clpid": "Kovalev-Y-Y" }, { "family_name": "Ros", "given_name": "E.", "orcid": "0000-0001-9503-4892", "clpid": "Ros-E" }, { "family_name": "Savolainen", "given_name": "T.", "orcid": "0000-0001-6214-1085", "clpid": "Savolainen-T" }, { "family_name": "Zensus", "given_name": "J. A.", "clpid": "Zensus-J-A" } ], "abstract": "We discuss the jet kinematics of a complete flux-density-limited sample of 135 radio-loud active galactic nuclei (AGNs) resulting from a 13 year program to investigate the structure and evolution of parsec-scale jet phenomena. Our analysis is based on new 2 cm Very Long Baseline Array (VLBA) images obtained between 2002 and 2007, but includes our previously published observations made at the same wavelength, and is supplemented by VLBA archive data. In all, we have used 2424 images spanning the years 1994-2007 to study and determine the motions of 526 separate jet features in 127 jets. The data quality and temporal coverage (a median of 15 epochs per source) of this complete AGN jet sample represent a significant advance over previous kinematics surveys. In all but five AGNs, the jets appear one-sided, most likely the result of differential Doppler boosting. In general, the observed motions are directed along the jet ridge line, outward from the optically thick core feature. We directly observe changes in speed and/or direction in one third of the well-sampled jet components in our survey. While there is some spread in the apparent speeds of separate features within an individual jet, the dispersion is about three times smaller than the overall dispersion of speeds among all jets. This supports the idea that there is a characteristic flow that describes each jet, which we have characterized by the fastest observed component speed. The observed maximum speed distribution is peaked at ~10c, with a tail that extends out to ~50c. This requires a distribution of intrinsic Lorentz factors in the parent population that range up to ~50. We also note the presence of some rare low-pattern speeds or even stationary features in otherwise rapidly flowing jets that may be the result of standing re-collimation shocks, and/or a complex geometry and highly favorable Doppler factor.", "doi": "10.1088/0004-6256/138/6/1874", "issn": "0004-6256", "publisher": "American Astronomical Society", "publication": "Astronomical Journal", "publication_date": "2009-12", "series_number": "6", "volume": "138", "issue": "6", "pages": "1874-1892" }, { "id": "authors:vxqe3-mwq26", "collection": "authors", "collection_id": "vxqe3-mwq26", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20091231-110730776", "type": "article", "title": "Genesis of the 1000-Foot Arecibo Dish", "author": [ { "family_name": "Cohen", "given_name": "Marshall H.", "orcid": "0000-0002-8033-5972", "clpid": "Cohen-M-H" } ], "abstract": "The giant radar/radio astronomy dish near Arecibo, Puerto Rico, was conceived by William E. Gordon in early 1958 as a back-scattering radar system to measure the density and temperature of the Earth's ionosphere up to a few thousand kilometers. Gordon calculated the required size of the antenna by using the Thomson cross-section for scattering by the electrons, and assuming that the elementary scattered waves would be incoherent. During the summer and autumn of 1958 Gordon led a study group that published a design report in December 1958. The report showed that a dish 1000 feet in diameter would be required, and described a limestone sinkhole in Puerto Rico that would make a suitable support for such a dish. Meanwhile, in November 1958, Kenneth L. Bowles per-formed an ionospheric radar experiment that showed that the Gordon calculation for the scattered power was roughly correct, but that the calculated spectral width was too big. The consequence of these results was that a dish substantially smaller than 1000 feet could have satisfied the original goals for the radar. However, from the spring of 1958 the value of 1000 feet had been in the minds of the study team, and a large suite of important experiments that such a dish could do had been identified. These apparently became the raison d'\u00eatre for the project, and the possibility of shrinking the dish to accomplish only the original goals seems to have been ignored. The project was sold to a new federal funding agency, the Advanced Research Projects Agency (ARPA), which was interested, in part at least, because ballistic missiles traveled through the ionosphere and it was important to fully understand that environment. Gordon's original calculation contained a remarkably beneficial error. Without it, it is doubtful that such a large dish would have been built.", "issn": "1440-2807", "publisher": "Centre for Astronomy, James Cook University", "publication": "Journal of Astronomical History and Heritage", "publication_date": "2009-07", "series_number": "2", "volume": "12", "issue": "2", "pages": "141-152" }, { "id": "authors:6g76r-ztb54", "collection": "authors", "collection_id": "6g76r-ztb54", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20090413-090043138", "type": "article", "title": "MOJAVE: monitoring of jets in active galactic nuclei with VLBA experiments. V. Multi-epoch VLBA images", "author": [ { "family_name": "Lister", "given_name": "M. L.", "orcid": "0000-0003-1315-3412", "clpid": "Lister-M-L" }, { "family_name": "Aller", "given_name": "H. D.", "clpid": "Aller-H-D" }, { "family_name": "Aller", "given_name": "M. F.", "orcid": "0000-0003-2483-2103", "clpid": "Aller-M-F" }, { "family_name": "Cohen", "given_name": "M. H.", "orcid": "0000-0002-8033-5972", "clpid": "Cohen-M-H" }, { "family_name": "Homan", "given_name": "D. C.", "orcid": "0000-0002-4431-0890", "clpid": "Homan-D-C" }, { "family_name": "Kadler", "given_name": "M.", "clpid": "Kadler-M" }, { "family_name": "Kellermann", "given_name": "K. I.", "orcid": "0000-0002-0093-4917", "clpid": "Kellermann-K-I" }, { "family_name": "Kovalev", "given_name": "Y. Y.", "orcid": "0000-0001-9303-3263", "clpid": "Kovalev-Y-Y" }, { "family_name": "Ros", "given_name": "E.", "orcid": "0000-0001-9503-4892", "clpid": "Ros-E" }, { "family_name": "Savolainen", "given_name": "T.", "orcid": "0000-0001-6214-1085", "clpid": "Savolainen-T" }, { "family_name": "Zensus", "given_name": "J. A.", "clpid": "Zensus-J-A" }, { "family_name": "Vermeulen", "given_name": "R. C.", "clpid": "Vermeulen-R-C" } ], "abstract": "We present images from a long-term program (MOJAVE: Monitoring of Jets in active galactic nuclei (AGNs) with VLBA Experiments) to survey the structure and evolution of parsec-scale jet phenomena associated with bright radio-loud active galaxies in the northern sky. The observations consist of 2424 15 GHz Very Long Baseline Array (VLBA) images of a complete flux-density-limited sample of 135 AGNs above declination \u201320\u00b0, spanning the period 1994 August to 2007 September. These data were acquired as part of the MOJAVE and 2 cm Survey programs, and from the VLBA archive. The sample-selection criteria are based on multi-epoch parsec-scale (VLBA) flux density, and heavily favor highly variable and compact blazars. The sample includes nearly all the most prominent blazars in the northern sky, and is well suited for statistical analysis and comparison with studies at other wavelengths. Our multi-epoch and stacked-epoch images show 94% of the sample to have apparent one-sided jet morphologies, most likely due to the effects of relativistic beaming. Of the remaining sources, five have two-sided parsec-scale jets, and three are effectively unresolved by the VLBA at 15 GHz, with essentially all of the flux density contained within a few tenths of a milliarcsecond.", "doi": "10.1088/0004-6256/137/3/3718", "issn": "0004-6256", "publisher": "American Astronomical Society", "publication": "Astronomical Journal", "publication_date": "2009-03", "series_number": "3", "volume": "137", "issue": "3", "pages": "3718-3729" }, { "id": "authors:k3ajd-qg454", "collection": "authors", "collection_id": "k3ajd-qg454", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20100923-111741692", "type": "article", "title": "Doppler boosting, superluminal motion, and the kinematics of AGN jets", "author": [ { "family_name": "Kellermann", "given_name": "K. I.", "orcid": "0000-0002-0093-4917", "clpid": "Kellermann-K-I" }, { "family_name": "Kovalev", "given_name": "Y. Y.", "orcid": "0000-0001-9303-3263", "clpid": "Kovalev-Y-Y" }, { "family_name": "Lister", "given_name": "M. L.", "orcid": "0000-0003-1315-3412", "clpid": "Lister-M-L" }, { "family_name": "Homan", "given_name": "D. C.", "orcid": "0000-0002-4431-0890", "clpid": "Homan-D-C" }, { "family_name": "Kadler", "given_name": "M.", "clpid": "Kadler-M" }, { "family_name": "Cohen", "given_name": "M. H.", "orcid": "0000-0002-8033-5972", "clpid": "Cohen-M-H" }, { "family_name": "Ros", "given_name": "E.", "orcid": "0000-0001-9503-4892", "clpid": "Ros-E" }, { "family_name": "Zensus", "given_name": "J. A.", "clpid": "Zensus-J-A" }, { "family_name": "Vermeulen", "given_name": "R. C.", "clpid": "Vermeulen-R-C" }, { "family_name": "Aller", "given_name": "M. F.", "orcid": "0000-0003-2483-2103", "clpid": "Aller-M-F" }, { "family_name": "Aller", "given_name": "H. D.", "clpid": "Aller-H-D" } ], "abstract": "We discuss results from a decade long program to study the fine-scale structure and the kinematics of relativistic AGN jets with the aim of better understanding the acceleration and collimation of the relativistic plasma forming AGN jets. From the observed distribution of brightness temperature, apparent velocity, flux density, time variability, and apparent luminosity, the intrinsic properties of the jets including Lorentz factor, luminosity, orientation, and brightness temperature are discussed. Special attention is given to the jet in M87, which has been studied over a wide range of wavelengths and which, due to its proximity, is observed with excellent spatial resolution.\nMost radio jets appear quite linear, but we also observe curved non-linear jets and non-radial motions. Sometimes, different features in a given jet appear to follow the same curved path but there is evidence for ballistic trajectories as well. The data are best fit with a distribution of Lorentz factors extending up to \u03b3\u223c30 and intrinsic luminosity up to ~10^(26) W \u2009Hz^(\u22121). In general, gamma-ray quasars may have somewhat larger Lorentz factors than non gamma-ray quasars. Initially the observed brightness temperature near the base of the jet extend up to ~5\u00d710^(13) K which is well in excess of the inverse Compton limit and corresponds to a large excess of particle energy over magnetic energy. However, more typically, the observed brightness temperatures are ~2\u00d710^(11) K, i.e., closer to equipartition.", "doi": "10.1007/s10509-007-9622-5", "issn": "0004-640X", "publisher": "Springer", "publication": "Astrophysics and Space Science", "publication_date": "2007-10", "series_number": "1-3", "volume": "311", "issue": "1-3", "pages": "231-239" }, { "id": "authors:76j51-7n149", "collection": "authors", "collection_id": "76j51-7n149", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:HOMapjl06", "type": "article", "title": "Intrinsic brightness temperatures of AGN jets", "author": [ { "family_name": "Homan", "given_name": "D. C.", "orcid": "0000-0002-4431-0890", "clpid": "Homan-D-C" }, { "family_name": "Kovalev", "given_name": "Y. Y.", "orcid": "0000-0001-9303-3263", "clpid": "Kovalev-Y-Y" }, { "family_name": "Lister", "given_name": "M. L.", "orcid": "0000-0003-1315-3412", "clpid": "Lister-M-L" }, { "family_name": "Ros", "given_name": "E.", "orcid": "0000-0001-9503-4892", "clpid": "Ros-E" }, { "family_name": "Kellermann", "given_name": "K. I.", "orcid": "0000-0002-0093-4917", "clpid": "Kellermann-K-I" }, { "family_name": "Cohen", "given_name": "M. H.", "orcid": "0000-0002-8033-5972", "clpid": "Cohen-M-H" }, { "family_name": "Vermeulen", "given_name": "R. C.", "clpid": "Vermeulen-R-C" }, { "family_name": "Zensus", "given_name": "J. A.", "clpid": "Zensus-J-A" }, { "family_name": "Kadler", "given_name": "M.", "clpid": "Kadler-M" } ], "abstract": "We present a new method for studying the intrinsic brightness temperatures of the parsec-scale jet cores of active galactic nuclei (AGNs). Our method uses observed superluminal motions and observed brightness temperatures for a large sample of AGNs to constrain the characteristic intrinsic brightness temperature of the sample as a whole. To study changes in intrinsic brightness temperature, we assume that the Doppler factors of individual jets are constant in time, as justified by their relatively small changes in observed flux density. We find that in their median-low brightness temperature state, the sources in our sample have a narrow range of intrinsic brightness temperatures centered on a characteristic temperature, Tint_~ 3 \u00d7 10 to the 10th K, which is close to the value expected for equipartition, when the energy in the radiating particles equals the energy stored in the magnetic fields. However, in their maximum brightness state, we find that sources in our sample have a characteristic intrinsic brightness temperature greater than 2 \u00d7 10 to the 11th K, which is well in excess of the equipartition temperature. In this state, we estimate that the energy in radiating particles exceeds the energy in the magnetic field by a factor of ~ 10 to the 5th. We suggest that the excess of particle energy when sources are in their maximum brightness state is due to injection or acceleration of particles at the base of the jet. Our results suggest that the common method of estimating jet Doppler factors by using a single measurement of observed brightness temperature, the assumption of equipartition, or both may lead to large scatter or systematic errors in the derived values.", "issn": "0004-637X", "publisher": "Astrophysical Journal", "publication": "Astrophysical Journal", "publication_date": "2006-05-10", "series_number": "2", "volume": "642", "issue": "2", "pages": "L115-L118" }, { "id": "authors:abn04-exp92", "collection": "authors", "collection_id": "abn04-exp92", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20180209-084634866", "type": "article", "title": "The shroud around the twin radio jets in NGC 1052", "author": [ { "family_name": "Vermeulen", "given_name": "R. C.", "clpid": "Vermeulen-R-C" }, { "family_name": "Ros", "given_name": "E.", "orcid": "0000-0001-9503-4892", "clpid": "Ros-E" }, { "family_name": "Kellermann", "given_name": "K. I.", "orcid": "0000-0002-0093-4917", "clpid": "Kellermann-K-I" }, { "family_name": "Cohen", "given_name": "M. H.", "orcid": "0000-0002-8033-5972", "clpid": "Cohen-M-H" }, { "family_name": "Zensus", "given_name": "J. A.", "clpid": "Zensus-J-A" }, { "family_name": "van Langevelde", "given_name": "H. J.", "clpid": "van-Langevelde-H-J" } ], "abstract": "We discuss multiple Very Long Baseline Interferometry (VLBI) continuum and spectral line imaging observations and Westerbork Synthesis Radio Telescope spectroscopy of the compact variable nuclear radio jet source in the elliptical galaxy NGC 1052. Absorption and emission signatures reveal ionised, atomic, and molecular components of the surrounding medium.\nTen epochs of Very Long Baseline Array (VLBA) data at 15 GHz, spanning almost six years, show bi-symmetric jets, in which multiple sub-parsec scale features display outward motions of typically v_(app) ~ 0.26c(H_0km s^(-1) Mpc^(-1)) on each side. The jets are most likely oriented near the plane of the sky.\nMulti-frequency VLBA observations at seven frequencies between 43 and 1.4 GHz show free-free absorption in the inner parsec around the nucleus, probably together with synchrotron self-absorption. The free-free absorption is apparently due to a structure which is geometrically thick and oriented roughly orthogonal to the jets, but which is patchy. The western jet is covered more deeply and extensively, and hence is receding.\nHi spectral line VLBI observations reveal atomic gas in front of the approaching as well as the receding jet. There appear to be three velocity systems. Broad, shallow absorption asymmetrically straddles the systemic velocity spanning -35 to 85 km s^(-1). This gas could be local to the AGN environment, or distributed on galactic scales. Superimposed in the range 25 to 95 km s^(-1) are several sharper (3\u201315 km s^(-1)) features, each detectable over a few tenths of a pc at various places along the inner 2 pc of the approaching jet. The third, deepest system is at \"high velocities\", which is receding by 125 to 200 km s^(-1) with respect to the systemic velocity of NGC 1052. It may have a continuous velocity gradient across the nucleus of some 10 km s^(-1) pc^(-1). This atomic gas seems restricted to a shell 1\u20132 pc away from the core, within which it might be largely ionised.\n Westerbork Synthesis Radio Telescope spectroscopy has revealed the 18 cm OH main lines (1667 and 1665 MHz) in absorption along the full velocity span of -35 to 200 km s^(-1), with their line ratio varying roughly from 1:1 to 2:1. They are deepest in the high velocity system, where the OH profiles are similar to Hi, suggesting co-location of that atomic and molecular gas, and leaving unclear the connection to the H_2O masing gas seen elsewhere. In the high velocity system we have also detected the 18 cm OH satellite lines: 1612 MHz in absorption, and 1720 MHz in emission. The conjugate behaviour of the satellite line profiles, and the variable main line ratio resemble the situation in Cen A and NGC 253.", "doi": "10.1051/0004-6361:20021752", "issn": "0004-6361", "publisher": "EDP Sciences", "publication": "Astronomy & Astrophysics", "publication_date": "2003-03-03", "series_number": "1", "volume": "401", "issue": "1", "pages": "113-127" }, { "id": "authors:8s1hj-65696", "collection": "authors", "collection_id": "8s1hj-65696", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20170408-160532910", "type": "article", "title": "Sub-milliarcsecond Imaging of Quasars and Active Galactic Nuclei. II. Additional Sources", "author": [ { "family_name": "Zensus", "given_name": "J. A.", "clpid": "Zensus-J-A" }, { "family_name": "Ros", "given_name": "E.", "orcid": "0000-0001-9503-4892", "clpid": "Ros-E" }, { "family_name": "Kellermann", "given_name": "K. I.", "orcid": "0000-0002-0093-4917", "clpid": "Kellermann-K-I" }, { "family_name": "Cohen", "given_name": "M. H.", "orcid": "0000-0002-8033-5972", "clpid": "Cohen-M-H" }, { "family_name": "Vermeulen", "given_name": "R. C.", "clpid": "Vermeulen-R-C" }, { "family_name": "Kadler", "given_name": "M.", "clpid": "Kadler-M" } ], "abstract": "We report further results from our imaging survey at 15 GHz (\u03bb = 2 cm) with the Very Long Baseline Array (VLBA). This paper presents single-epoch images for 39 sources, bringing the total number of objects in the sample to 171. Our sample is representative of a complete unbiased sample, and it will be used for statistical analysis of source properties. We compare the observed brightness temperatures derived from our VLBA observations with those derived from total intensity variations at 22 and 37 GHz. These are consistent with intrinsic brightness temperatures in the range 10^(10)\u201310(12) K. We also present three new spectroscopic redshift values: z = 0.517 \u00b1 0.001 for 0026+346, z = 1.591 \u00b1 0.003 for 0727-115, and z = 0.2016 \u00b1 0.0004 for 1155+251.", "doi": "10.1086/341585", "issn": "1538-3881", "publisher": "American Astronomical Society", "publication": "Astronomical Journal", "publication_date": "2002-08", "series_number": "2", "volume": "124", "issue": "2", "pages": "662-674" }, { "id": "authors:6vcsa-08r49", "collection": "authors", "collection_id": "6vcsa-08r49", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20180207-090718468", "type": "article", "title": "Radio galaxies at z ~ 2.5: Results from Keck spectropolarimetry", "author": [ { "family_name": "Vernet", "given_name": "J.", "clpid": "Vernet-J" }, { "family_name": "Fosbury", "given_name": "R. A. E.", "clpid": "Fosbury-R-A-E" }, { "family_name": "Villar-Mart\u00edn", "given_name": "M.", "clpid": "Villar-Mart\u00edn-M" }, { "family_name": "Cohen", "given_name": "M. H.", "orcid": "0000-0002-8033-5972", "clpid": "Cohen-M-H" }, { "family_name": "Cimatti", "given_name": "A.", "clpid": "Cimatti-A" }, { "family_name": "di Serego Alighieri", "given_name": "S.", "orcid": "0000-0001-8769-2692", "clpid": "di-Serego-Alighieri-S" }, { "family_name": "Goodrich", "given_name": "R. W.", "clpid": "Goodrich-R-W" } ], "abstract": "In classifying the ensemble of powerful extragalactic radio sources, considerable evidence has accumulated that radio galaxies and quasars are orientation-dependent manifestations of the same parent population: massive spheroidal galaxies containing correspondingly massive black holes. One of the key factors in establishing this unification has been the signature of a hidden quasar detected in some radio galaxies in polarized light. The obscuration of our direct view of the active nucleus usually, but not necessarily exclusively, by a thick nuclear disk or torus can act conveniently as a \"natural coronograph\" that allows a much clearer view of the host of a radio galaxy than of a quasar. In this study, we exploit the opportunity to eliminate the quasar glare by performing sensitive spectropolarimetry with the Keck II telescope of a sample of radio galaxies with redshifts around 2.5. This represents the epoch when quasars were many times more common that they are now and is likely to be the period during which their host galaxies were being assembled into what become the most massive galaxies in the Universe today. We show that dust-reflected quasar light generally dominates the restframe ultraviolet continuum of these sources and that a highly clumped scattering medium results in almost grey scattering of the active galactic nucleus photons. The observations, however, do not exclude a substantial star formation rate averaged over a Gyr of evolution. The sub-mm reradiation from the scattering dust is likely to represent only a small fraction (\u223c10% ) of the total far infrared luminosity. An analysis of the emission lines excited in the interstellar medium of the host galaxy by the hard quasar radiation field reveals evidence of a dramatic chemical evolution within the spheroid during this epoch. Secondary nitrogen production in intermediate mass stars produces a characteristic signature in the Nv/Civ and Nv/Heii line ratios which has been seen previously in the broad line region of quasars at similar redshifts. We find intriguing correlations between the strengths of the Ly\u03b1 and N v emission lines and the degree of ultraviolet continuum polarization which may represent the dispersal of dust associated with the chemical enrichment of the spheroid.", "doi": "10.1051/0004-6361:20000076", "issn": "0004-6361", "publisher": "EDP Sciences", "publication": "Astronomy & Astrophysics", "publication_date": "2001-01-15", "series_number": "1", "volume": "366", "issue": "1", "pages": "7-25" }, { "id": "authors:vvvvw-bpn09", "collection": "authors", "collection_id": "vvvvw-bpn09", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20141209-084132337", "type": "article", "title": "Quasars and active galactic nuclei: High resolution radio imaging", "author": [ { "family_name": "Cohen", "given_name": "Marshall H.", "orcid": "0000-0002-8033-5972", "clpid": "Cohen-M-H" }, { "family_name": "Kellerman", "given_name": "Kenneth I.", "clpid": "Kellerman-K-I" } ], "abstract": "The 12th colloquium in the series organized by the National\nAcademy of Sciences, Quasars and Active Galactic Nuclei: High Resolution Radio Imaging, was held at the NAS Beckman\nCenter in Irvine, California, on March 24 and 25, 1995. The\nmeeting brought together 138 scientists from 17 countries. The majority of the participants were radio astronomers, the colloquial name for astronomers who specialize in studies at radio wavelengths. However, there was also a strong mixture of theorists and other astronomers who work primarily in other wavelength bands, and this provided lively discussion and an occasional healthy dose of skepticism.", "doi": "10.1073/pnas.92.25.11339", "pmcid": "PMC40395", "issn": "0027-8424", "publisher": "National Academy of Sciences", "publication": "Proceedings of the National Academy of Sciences of the United States of America", "publication_date": "1995-12-05", "series_number": "25", "volume": "92", "issue": "25", "pages": "11339-11341" }, { "id": "authors:0djsm-p9359", "collection": "authors", "collection_id": "0djsm-p9359", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20170809-164825681", "type": "article", "title": "Introduction to very-long-baseline interferometry", "author": [ { "family_name": "Cohen", "given_name": "Marshall H.", "orcid": "0000-0002-8033-5972", "clpid": "Cohen-M-H" } ], "abstract": "Long-baseline interferometry achieves high resolution by using two or more widely separated radio telescopes and recording video signals on magnetic tapes, which are later brought together and cross-correlated. This paper contains discussions of the coherence and timing requirements and of calibration procedures. Applications to measuring brightness distributions and to spectroscopy are reviewed briefly. Some pertinent phenomena connected with radio-wave scattering in irregular media are discussed.", "doi": "10.1109/PROC.1973.9244", "issn": "0018-9219", "publisher": "IEEE", "publication": "Proceedings of the IEEE", "publication_date": "1973-09", "series_number": "9", "volume": "61", "issue": "9", "pages": "1192-1197" }, { "id": "authors:bf6yt-11n40", "collection": "authors", "collection_id": "bf6yt-11n40", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20161019-082031869", "type": "article", "title": "High-Resolution Observations of Radio Sources", "author": [ { "family_name": "Cohen", "given_name": "Marshall H.", "orcid": "0000-0002-8033-5972", "clpid": "Cohen-M-H" } ], "abstract": "Recent developments have greatly increased the angular resolution attainable in radio astronomy. The largest reflectors give a resolution of 2 to 3 min of are, close to that of the unaided human eye. Interplanetary scintillations and interferometry give 10^(-3) sec of arc and higher; this exceeds the resolution of Michelson's stellar interferometer by more than an order of magnitude, and is comparable to the highest resolution attained by the optical intensity interferometer (1).", "doi": "10.1146/annurev.aa.07.090169.003155", "issn": "0066-4146", "publisher": "Annual Reviews", "publication": "Annual Review of Astronomy and Astrophysics", "publication_date": "1969-09", "volume": "7", "pages": "619-664" } ]