X-ray emission from radio-quiet quasars in the SDSS Early Data Release. The alpha_ox dependence upon UV luminosity

Abstract

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, alpha_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 alpha_ox is a function of rest-frame UV luminosity (i.e., alpha_ox 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 alpha_ox. We also find a significant (7.8 sigma level) correlation between UV and X-ray luminosity. This correlation, parameterized by Lx \propto Luv^{0.75+/-0.06}, extends previous results to the highest redshifts.