SIMBAD references

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<νL_ν(2500 Å)<10⁴⁸ ergs/s. 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_oxon cosmic time and a quadratic dependence of α_ox on logL_UV, where α_oxbecomes X-ray quiet more rapidly at higher logL_UV. We find no significant evidence for a relationship between the X-ray photon index Γ_xand the UV luminosity, and we find marginally significant evidence that the X-ray continuum flattens with increasing z (2 σ). The Γ_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 Γ_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_oxand z implies evolution of the accretion process. We present a qualitative comparison of these new results with models for accretion disk emission.