Evolution of the X-ray Emission of Radio-Quiet Quasars
Abstract
We report new Chandra observations of seven optically faint, z 4 radio-quiet quasars. We have combined these new observations with previous Chandra observations of radio-quiet quasars to create a sample of 174 sources. These sources have 0.1 < z < 4.7, and 1044 ergs s-1 < nu Lnu (2500 ) < 1048 ergs s-1. The X-ray detection fraction is 90%. We find that the X-ray loudness of radio-quiet quasars 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, alphaox, on cosmic time, and a quadratic dependence of alphaox on log LUV, where alphaox becomes X-ray quiet more rapidly at higher log LUV. We find no significant evidence for a relationship between the X-ray photon index, GammaX, and the UV luminosity, and we find marginally significant evidence that the X-ray continuum flattens with increasing z (2 sigma). The GammaX-z anti-correlation 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 GammaX, we show that the alphaox-z relationship is unlikely to be a spurious result caused by redshifting of the observable X-ray spectral region. A correlation between alphaox and z implies evolution of the accretion process. We present a qualitative comparison of these new results with models for accretion disk emission.
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