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

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 10⁴⁴ ergs s^-1 < νL_ν (2500 Å) < 10⁴⁸ 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 α_ox on cosmic time and a quadratic dependence of α_ox on log L_UV, where α_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 Fx and the UV luminosity, and we find marginally significant evidence that the X-ray continuum flattens with increasing z (2 σ). The Τ_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 Τ_x, we show that the α_ox-z relationship is unlikely to be a spurious result caused by redshifting of the observable X-ray spectral region. A correlation between α_ox and z implies evolution of the accretion process. We present a qualitative comparison of these new results with models for accretion disk emission.