X-ray emission from radio-quiet quasars in the Sloan Digital Sky Survey Early Data Release: The α_OX dependence upon ultraviolet luminosity

We investigate the X-ray properties of the color-selected, radio-quiet quasars (RQQs) in the Sloan Digital Sky Survey (SDSS) Early Data Release using ROSAT, Chandra, and XMM-Newton data. In the 0.16-6.28 redshift range, 136 RQQs have X-ray detections (69 from the ROSAT All-Sky Survey, RASS), while for 70 RQQs X-ray upper limits are obtained. The well-defined selection method used by the SDSS, coupled with the tight radio constraints from the FIRST and NVSS surveys, allows us to define a representative sample of optically selected RQQs whose broadband spectral energy distributions (characterized by means of the optical-to-X-ray spectral index, α_OX) can be studied as a function of rest-frame ultraviolet (UV) luminosity and redshift. A partial correlation analysis applied to the SDSS sample (including the upper limits, but excluding the biased subsample of RASS detections) shows that α _OX is a function of rest-frame UV luminosity (i.e., α _OX steepens at high UV luminosities); this correlation is significant at the 3.7 σ level. We do not detect a highly significant redshift dependence of α_OX. We also find a significant (7.8 σ level) correlation between UV and X-ray luminosity. This correlation, parameterized by L_X ∝ L_UV ^0.75±0.06, extends previous results to the highest redshifts.