X-Ray Emission from Radio-quiet Quasars in the Sloan Digital Sky Survey Early Data Release: The αTINFCLCox/CLC/TINF 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 utilized by the SDSS, coupled with the tight radio constraints from the FIRST/NVSS surveys, allow us to define a representative sample of optically selected RQQs whose broad-band spectral energy distributions (characterized by means of the optical-to-X-ray spectral index, alphaₒx) 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 alphaₒx is a function of rest-frame UV luminosity (i. e. , alphaₒx steepens at high UV luminosities) ; this correlation is significant at the 3. 7 sigma level. We do not detect a highly significant redshift dependence of alphaₒx. We also find a significant (7. 8 sigma level) correlation between UV and X-ray luminosity. This correlation, parameterized by Lx Luv^0. 75+/-0. 06, extends previous results to the highest redshifts.