Book Description

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.