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Date and Time of the Query: 2018-12-11 T04:34:24 PST
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For refcode 2007ApJ...657..116K:
Retrieve 174 NED objects in this reference.
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Copyright by American Astronomical Society. Reproduced by permission
2007ApJ...657..116K Evolution of the X-ray Emission of Radio-quiet Quasars Brandon C. Kelly, Jill Bechtold, Aneta Siemiginowska, Tom Aldcroft, and Malgorzata Sobolewska Abstract. We report new Chandra observations of seven optically faint, z~4 radio-quiet quasars (RQQs). We have combined these new observations with previous Chandra observations of RQQs to create a sample of 174 sources. These sources have 0.1<z<4.7, and 10^44^ergss^-1^<{nu}L_{nu}_(2500Angstrom)<10^48^ergss^-1^. The X-ray detection fraction is 90%. We find that the X-ray loudness of RQQs decreases with UV luminosity and increases with redshift. The model that is best supported by the data has a linear dependence of optical to X-ray ratio {alpha}_ox_ on cosmic time and a quadratic dependence of {alpha}_ox_ on log L_UV_, where {alpha}_ox_ becomes X-ray quiet more rapidly at higher log L_UV_. We find no significant evidence for a relationship between the X-ray photon index {GAMMA}_x_ and the UV luminosity, and we find marginally significant evidence that the X-ray continuum flattens with increasing z (2 {sigma}). The {GAMMA}_x_-z anticorrelation may be the result of X-ray spectral curvature, redshifting of a Compton reflection component into the observed Chandra band, and/or redshifting of a soft excess out of the observed Chandra band. Using the results for {GAMMA}_x_, we show that the {alpha}_ox_-z relationship is unlikely to be a spurious result caused by redshifting of the observable X-ray spectral region. A correlation between {alpha}_ox_ and z implies evolution of the accretion process. We present a qualitative comparison of these new results with models for accretion disk emission. Key words: Accretion, Accretion Disks, Galaxies: Quasars: General, Ultraviolet: Galaxies, X-Rays: Galaxies
Retrieve 174 NED objects in this reference.
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