Evolution of the X-ray emission of radio-quiet quasars

We report new Chandra observations of seven optically faint, z similar to 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) ergs s(-1) < nu L-nu (2500 angstrom) < 10(48) ergs s(-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 findmarginally 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.