THE SLOAN DIGITAL SKY SURVEY/XMM-NEWTON QUASAR SURVEY: CORRELATION BETWEEN X-RAY SPECTRAL SLOPE AND EDDINGTON RATIO

We present a correlation between the 2-10 keV spectral slope Gamma(X) and the Eddington ratio L/L(EDD) in a sample of similar to 400 Sloan Digital Sky Survey (SDSS) quasars with available hard X-ray spectra from XMM-Newton serendipitous observations. We find that the Gamma(X)-L/L(EDD) correlation is strongest in objects with black hole (BH) masses determined from the H beta line, and weaker (but still present) for those based on Mg II. An empirical nonlinear correction of the Mg II-based masses, obtained by comparing the mass estimates in SDSS quasars having both H beta and Mg II measurements, significantly increases the strength of the correlation. No correlation is found among objects with BH masses derived from C IV, confirming that this line is not a reliable indicator of the BH mass. No significant correlation is found with the bolometric luminosity, while a Gamma(X)-M(BH) relation is present, though with a lower statistical significance than between Gamma(X) and L/L(EDD). Our results imply a physical link between the accretion efficiency in the (cold) accretion disk of active galactic nuclei and the physical status of the (hot) corona responsible for the X-ray emission.