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The aim of this research was to use the X-ray satellite Suzaku to establish a picture of a central engine that effectively converts the gravitational energy of accreting matter onto the supermassive black hole to a huge amount of radiation in an active galactic nucleus. Although the engine is known to consist of a Comptonizing corona and an accretion disk, its image has remained unclear because primary emissions, coming directly from the engine, cannot be identified in X-ray spectra without models. The book describes a technique of time variability assisted spectral decomposition to model-independently examine X-ray signals, and how this was applied to the Suzaku archive data of active galactic nuclei. As a result, at least three distinct primary X-ray components have been discovered in an X-ray from an active galactic nucleus, presumably indicating a novel picture that the engine is composed of multiple coronae with different physical properties in an accretion flow. Furthermore, the determination of the spectral shapes of the primary X-rays has a significant impact on estimations of black hole spins, because it is essential to quantify reprocessed X-ray spectra. The successful model-independent decomposition of X-ray spectral components with flux variations of active galactic nuclei is likely to be effective in future data analyses from the soon-to-be-launched Japanese X-ray satellite ASTRO-H, which is capable of achieving unprecedented fine spectros copy and broad energy band coverage.
Active galactic nuclei (AGNs) are ideal laboratories for fundamental physics and cosmology. Our knowledge of AGNs has been greatly advanced during the past decades thanks to the unprecedentedly powerful X-ray missions (e.g., Chandra, XMM-Newton, and Swift) and modern large area surveys in the optical/UV band (e.g., the Sloan Digital Sky Survey; SDSS). Studies on the relation between multiwavelength properties of AGNs can reveal the physics of AGN accretion process. In this dissertation, I mainly study the relation between the AGN X-ray properties and their optical/UV spectral properties on emission lines, absorption lines and continuum. I also investigate the X-ray properties of AGNs with extreme radio properties in the early Universe. (1) We studied the X-ray properties of a class of radio-quiet SDSS quasars with weak broad emission lines (weak-line quasars; WLQs). Although the WLQ population shows diverse X-ray properties, they have an excess of X-ray weak sources. Besides having weak emission lines, the X-ray weak WLQs generally show other unusual UV emission-line properties similar to those of the remarkable X-ray weak quasar PHL 1811 (e.g., highly blueshifted C IV lines, weak semi-forbidden lines, and strong UV Fe emission). They are classified as "PHL 1811 analogs". The X-ray weak WLQs also show a harder X-ray spectrum, while the WLQ with normal X-ray brightness have similar X-ray spectral properties to those of typical quasars. We proposed an AGN geometry which can potentially unify the X-ray weak and X-ray normal WLQ populations via orientation effect. The infrared-to-UV spectral energy distributions (SEDs) of X-ray weak and X-ray normal WLQs have consistent SEDs with those for typical quasars, which disfavors the BL Lac-like scenario for the nature of these quasars (Chapters 2 and 3). (2) We have led the best X-ray study to date on quasars with intermediate-width absorption lines (mini-BALs). We found the X-ray brightness of mini-BAL quasars are more close to those of typical quasars than to BAL quasars (which are generally X-ray weak), showing they do not have substantial X-ray absorption. Strong correlations were found between the X-ray brightness and UV absorption parameters, e.g., the absorption strength and maximum outflow velocity. We further proposed new UV absorption parameters which better correlate with the X-ray properties than existing parameters do (Chapter 4). (3) We studied the relation between the optical/UV luminosity and X-ray luminosity (quantified by the alpha_ox parameter) for the most-luminous quasars over a wide range of redshifts. Our correlation analyses provide better constraints on the alpha_ox-UV luminosity correlation. We have also verified that the alpha_ox parameter does not significantly evolve with redshift. We provide the individual and composite mid-infrared-to-UV SEDs for the most-luminous quasars. (Chapter 5). (4) We presented the X-ray and multiwavelength properties of the highly radio-loud quasars (HRLQs) at z > 4. Our HRLQs show a significant enhancement of X-ray emission over those HRLQs at lower redshift with similar optical/UV and radio luminosities, suggesting that the jet-linked X-ray emission mechanism in the early universe may differ from that in the more evolved universe. The optical/UV emission-line strength of RLQs are correlated with radio loudness, but not with relative X-ray brightness. Our HRLQs generally follow the anti-correlation between radio loudness and X-ray power-law photon index. We also studied the broad-band SEDs of HRLQs. Some HRLQs have an excess of mid-infrared emission which may originate from the jet synchrotron emission. None of our z > 4 HRLQs is detected by the Fermi LAT two-year survey (Chapter 6).
How can we test if a supermassive black hole lies at the heart of every active galactic nucleus? What are LINERS, BL Lacs, N galaxies, broad-line radio galaxies and radio-quiet quasars and how do they compare? This timely textbook answers these questions in a clear, comprehensive and self-contained introduction to active galactic nuclei - for graduate students in astronomy and physics. The study of AGN is one of the most dynamic areas of contemporary astronomy, involving one fifth of all research astronomers. This textbook provides a systematic review of the observed properties of AGN across the entire electromagnetic spectrum, examines the underlying physics, and shows how the brightest AGN, quasars, can be used to probe the farthest reaches of the Universe. This book serves as both an entry point to the research literature and as a valuable reference for researchers in the field.
A review of the current observational knowledge and understanding of the cosmic X-ray background.
Observations of starburst galaxies have revealed a large number of point-like X-ray sources located within these systems, including active galactic nuclei (AGN), X-ray binaries (XRBs), and utraluminous X-ray sources (ULXs). In this thesis we investigate the physical properties of a variety of these objects using both their X-ray emission characteristics and their optical counterparts, in order to better understand the accretion physics of such objects, and the environments in which they are found. This work begins with a study of 3 moderate-redshift (z ~ 0.1) X-ray bright (> 1042 erg s−1) galaxies, all of which display no clear signs of the presence of an AGN in the optical band. Given the high X-ray luminosities of these objects, they must either be the most X-ray luminous starburst galaxies known; or they must harbour a hidden AGN. We use new, pointed observations of the galaxies to determine their detailed X-ray characteristics, and demonstrate that each X-ray source is consistent with an AGN. The most likely explanation for the lack of AGN signatures in the optical spectra of these galaxies is that the AGN emission lines are being diluted by star formation signatures from within the host galaxies. Next, we present age constraints on 13 bright XRBs located within the high-metallicity drop-through ring galaxy NGC 922. While less than half the X-ray sources are located close to recent star formation as traced by H? emission, the majority of sources are associated with star clusters, that we are able to age on the basis of their optical colours. We find that the sources that are closest to clusters tend to also have the youngest counterpart clusters and, in most cases, are close to H$\alpha$ emission, placing a limit of