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""Changing-look" quasars are a new class of highly variable active galactic nuclei that have changed their spectral type over surprisingly short timescales of a few years.The origin of this phenomenon is currently debated, but is likely to reflect some change in the accretion flow. To investigate the disk-corona systems in these objects, I use new Chandra X-ray observations and optical spectroscopy to measure optical/UV-X-ray spectral indices and Eddington ratios of ten previously discovered changing-look quasars at two or more epochs. By comparing these data with simulated results for X-ray binaries undergoing accretion state transitions,I find a likely similarity in spectral indices below 1% Eddington ratio. This implies that the accretion state transition is a possible origin of changing-look quasars. I further investigate the Eddington ratios of changing-look quasars before and after their spectral type changes, and find that changing-look quasars cross the 1% Eddington ratio boundary when their broad emission lines disappear/emerge. This supports the disk-wind model as the origin of broad emission lines"--
The last 50 years have seen a tremendous progress in the research on quasars. From a time when quasars were unforeseen oddities, we have come to a view that considers quasars as active galactic nuclei, with nuclear activity a coming-of-age experienced by most or all galaxies in their evolution. We have passed from a few tens of known quasars of the early 1970s to the 500,000 listed in the catalogue of the Data Release 14 of the Sloan Digital Sky Survey. Not surprisingly, accretion processes on the central black holes in the nuclei of galaxies — the key concept in our understanding of quasars and active nuclei in general — have gained an outstanding status in present-day astrophysics. Accretion produces a rich spectrum of phenomena in all bands of the electromagnetic spectrum. The power output of highly-accreting quasars has impressive effects on their host galaxies. All the improvement in telescope light gathering and in computing power notwithstanding, we still miss a clear connection between observational properties and theory for quasars, as provided, for example, by the H-R diagram for stars. We do not yet have a complete self-consistent view of nuclear activity with predictive power, as we do for main-sequence stellar sources. At the same time quasars offer many “windows open onto the unknown". On small scales, quasar properties depend on phenomena very close to the black hole event horizon. On large scales, quasars may effect evolution of host galaxies and their circum-galactic environments. Quasars’ potential to map the matter density of the Universe and help reconstruct the Universe’s spacetime geometry is still largely unexploited. The times are ripe for a critical assessment of our present knowledge of quasars as accreting black holes and of their evolution across the cosmic time. The foremost aim of this research topic is to review and contextualize the main observational scenarios following an empirical approach, to present and discuss the accretion scenario, and then to analyze how a closer connection between theory and observation can be achieved, identifying those aspects of our understanding that are still on a shaky terrain and are therefore uncertain knowledge. This research topic covers topics ranging from the nearest environment of the black hole, to the environment of the host galaxies of active nuclei, and to the quasars as markers of the large scale structure and of the geometry of spacetime of the Universe. The spatial domains encompass the accretion disk, the emission and absorption regions, circum-nuclear starbursts, the host galaxy and its interaction with other galaxies. Systematic attention is devoted to some key problems that remain outstanding and are clearly not yet solved: the existence of two quasar classes, radio quiet and radio loud, and in general, the systematic contextualization of quasar properties the properties of the central black hole, the dynamics of the accretion flow in the inner parsecs and the origin of the accretion matter, the quasars’ small and large scale environment, the feedback processes produced by the black hole into the host galaxy, quasar evolutionary patterns from seed black holes to the present-day Universe, and the use of quasars as cosmological standard candles. The timing is appropriate as we are now witnessing a growing body of results from major surveys in the optical, UV X, near and far IR, and radio spectral domains. Radio instrumentation has been upgraded to linear detector — a change that resembles the introduction of CCDs for optical astronomy — making it possible to study radio-quiet quasars at radio frequencies. Herschel and ALMA are especially suited to study the circum-nuclear star formation processes. The new generation of 3D magnetohydrodynamical models offers the prospective of a full physical modeling of the whole quasar emitting regions. At the same time, on the forefront of optical astronomy, applications of adaptive optics to long-slit spectroscopy is yielding unprecedented results on high redshift quasars. Other measurement techniques like 2D and photometric reverberation mapping are also yielding an unprecedented amount of data thanks to dedicated experiments and instruments. Thanks to the instrumental advances, ever growing computing power as well as the coming of age of statistical and analysis techniques, the smallest spatial scales are being probed at unprecedented resolution for wide samples of quasars. On large scales, feedback processes are going out of the realm of single-object studies and are entering into the domain of issues involving efficiency and prevalence over a broad range of cosmic epochs. The Research Topic "Quasars at all Cosmic Epochs" collects a large fraction of the contributions presented at a meeting held in Padova, sponsored jointly by the National Institute for Astrophysics, the Padova Astronomical Observatory, the Department of Physics and Astronomy of the University of Padova, and the Instito de Astrofísica de Andalucía (IAA) of the Consejo Superiór de Investigación Cientifica (CSIC). The meeting has been part of the events meant to celebrate the 250th anniversary of the foundation of the Padova Observatory.
Abstract: Black holes, lurking at the centers of distant galaxies, feed on gas to become quasars. The bulk of the powerful emission from radio-quiet quasars originates from two structures close to the central black hole: the accretion disk and the hot corona. The disk-corona relationship and its dependence on the rate of gas accretion onto the central black hole is not well understood, in part because of the lack of knowledge regarding the corona temperature, density and geometry. To gain a better understanding of the accretion physics that affect the growth and evolution of quasars, as well as the evolution of their host galaxies, I perform a large-scale study of quasar optical through X-ray spectral energy distributions (SED). I have cross-correlated optical DR5 Sloan Digital Sky Survey (SDSS) quasars with the XMM-Newton archive of X-ray observations to obtain 792 optically-selected quasars with X-ray observations, 473 of which have X-ray spectra. I investigate relations between accretion rate, optical and X-ray luminosity, and X-ray slope. I compare the observed correlations with population synthesis simulations to determine which correlations are intrinsic to the physics of quasar accretion, and which are simply due to selection effects or the consequence of another correlation. I discuss the results with respect to physical models. At low accretion rates, the disk-corona structure may change significantly. I investigate this possibility in the case of red quasars by using optical and X-ray information to disentangle the effect of absorption and low accretion rates on red quasar SEDs. I find that 7 out of 17 of the reddest SDSS quasars are not well described by absorption. Instead, the red optical colors appear to be intrinsic to the accretion physics, and are perhaps related to the low accretion rates ( L/L Edd -0.05) observed for these objects. By extending the intrinsically red quasar SEDs to the infrared and ultraviolet wavelengths, I constrain standard disk models. By combining studies of large-scale trends with case studies of quasar accretion under extreme conditions, I investigate current models of disk-corona interactions.
I use X-ray and optical data to investigate the structure of quasars, and its dependence on luminosity, redshift, black hole mass, and Eddington ratio. In order to facilitate my work, I develop new statistical methods of accounting for measurement error, non-detections, and survey selection functions. The main results of this thesis follow. (1) The statistical uncertainty in the broad line mass estimates can lead to significant artificial broadening of the observed distribution of black hole mass. (2) The z = 0.2 broad line quasar black hole mass function falls off approximately as a power law with slope ̃2 for M_{BH}>108̂ M_{Sun}. (3) Radio-quiet quasars become more X-ray quiet as their optical/UV luminosity, black hole mass, or Eddington ratio increase, and more X-ray loud at higher redshift. These correlations imply that quasars emit a larger fraction of their bolometric luminosity through the accretion disk component, as compared to the corona component, as black hole mass and Eddington ratio increase. (4) The X-ray spectral slopes of radio-quiet quasars display a non-monotonic trend with Eddington ratio, where the X-ray continuum softens with increasing Eddington ratio until L / L_{Edd} ̃0.3, and then begins to harden. This observed non-monotonic trend may be caused by a change in the structure of the disk/corona system at L / L_{Edd} ̃0.3, possibly due to increased radiation pressure. (5) The characteristic time scales of quasar optical flux variations increase with increasing M_{BH}, and are consistent with disk orbital or thermal time scales. In addition the amplitude of short time scale variability decreases with increasing M_{BH}. I interpret quasar optical light curves as being driven by thermal fluctuations, which in turn are driven by some other underlying stochastic process with characteristic time scale long comparedto the disk thermal time scale. The stochastic model I use is able to explain both short and long time scale optical fluctuations.
Annotation Proceedings of a March 1998 meeting, with papers in sections on general considerations and photoionization modeling, profile variation and reverberation mapping, flows, disks, narrow-line Seyfert 1 galaxies, the intermediate line region and lower-velocity gas, X-ray warm absorber and dust, and unified theories and connections with extended emission. Specific topics include spectral variability of NGC 4151 during 1990 and disk emission line issues. Lacks a subject index. The editor is associated with the University of Nebraska. Annotation c. Book News, Inc., Portland, OR (booknews.com)
Thoroughly revised and expanded throughout, the new edition is a graduate-level text and reference book on gaseous nebulae, nova and supernova remnants. Much of the new data and new images are from the Hubble Space Telescope with two wholly new chapters being added along with other new features. The previous edition which was tried and tested for thirty years has now been succeeded by a revised, updated, larger edition, which will be valuable to anyone seriously interested in astrophysics.
Nearly simultaneous optical, ultraviolet, and x ray observations of three low redshift quasars are presented. The EXOSAT x ray spectra span the range of observed spectral indices for quasars from the canonical 0.7 energy index typical of Seyfert galaxies for PG0923+129 (Mrk 705) to the steep spectral indices frequently seen in higher luminosity quasars with an index of 1.58 for PG0844+349 (Ton 951). None of the quasars exhibits any evidence for a soft x ray excess. This is consistent with accretion disk spectra fit to the IR through UV continua of the quasars -- the best fitting disk spectra peak at approximately 6 eV with black hole masses in the range 5 x 10(exp 7) to 1 x 10(exp 9) solar mass and mass accretion rates of approximately 0.1 times the Eddington-limited rate. These rather soft disk spectra are also compatible with the observed optical and ultraviolet line ratios. Kriss, Gerard A. NASA-CR-186355, NAS 1.26:186355 NAG8-562...
Quasars are the ultra-luminous centers of some galaxies, powered by gas accreting onto a supermassive black hole after forming an accretion disk. The broad-line region of a quasar is the line-emitting region made up of fast-moving gas in the top layers of and around the accretion disk, which is photoionized by the hot inner disk. The emitting portion of the broad-line region (BLR) can change its effective size depending on the varying continuum luminosity of the quasar in a behavior called "breathing." In this thesis, I searched for breathing of the BLR of 534 quasars observed multiple times by the SDSS over the course of 17 years. I quantified the amount of breathing in the quasars by measuring changes in the broad H[beta] flux and line width. In quasars observed three or more times, I found that 14 quasars exhibited breathing, 24 quasars exhibited anti-breathing, and 47 quasars had ambiguous behavior. I considered quasars with only two observations as an ensemble, and found that they had a slight preference for anti-breathing. These results indicate that breathing of the broad H[beta] line is not universal, and other BLR behaviors such as anti-breathing are more common than once thought. The model of a BLR that consists of only virialized gas is insufficient, as it cannot explain the results found in this thesis. However, a two-component model for the BLR consisting of both virialized gas close to the accretion disk and a non-virial disk wind can account for the behaviors of quasars in my sample.