Nearly Simultaneous Optical, Ultraviolet, and X Ray Observations of Three Pg Quasars


Book Description

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...













Astrophysical Spectropolarimetry


Book Description

The polarization of light is the key to obtaining a wealth of essential information that lies encoded in the electromagnetic radiation from cosmic objects. Spectropolarimetry and imaging polarimetry provide powerful diagnostics of the physical conditions in astrophysical plasmas, which cannot be obtained via conventional spectroscopy. Whilst its application to other fields of astrophysics is still at an early stage of development, spectropolarimetry is being used with great success in solar physics. The book contains the lectures delivered at the XII Canary Islands Winter School of Astrophysics. Written by eight prestigious astrophysics researchers, it covers the physics of polarization, polarized radiation diagnostics of solar magnetic fields, stellar magnetic fields, polarization insights for active galactic nuclei, compact objects and accretion disks, astronomical masers and their polarization, infrared-submillimeter spectropolarimetry, and instrumentation for astrophysical spectropolarimetry. This timely volume will provide graduate students and researchers with an unprecedented introduction to the field of Astrophysical Spectropolarimetry.










Probing the Disk-corona Systems and the Broad Line Regions of Changing-look Quasars with Optical and X-ray Observations


Book Description

""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"--