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We present a correlation between the 2-10 keV spectral slope Gamma and theEddington ratio L/L_{EDD} in a sample of ~400 Sloan Digital Sky Survey quasarswith available hard X-ray spectra from XMM-Newton serendipitous observations.We find that the Gamma-L/L_{EDD} correlation is strongest in objects with blackhole (BH) masses determined from the Hbeta line, and weaker (but still present)for those based on Mg II. An empirical non-linear correction of the Mg II-basedmasses, obtained by comparing the mass estimates in SDSS quasars having bothHbeta and Mg II measurements, significantly increases the strength of thecorrelation. No correlation is found among objects with BH masses derived fromC IV, confirming that this line is not a reliable indicator of the BH mass. Nosignificant correlation is found with the bolometric luminosity, while aGamma-M_BH relation is present, though with a lower statistical significancethan between Gamma and L/L_{EDD}. Our results imply a physical link between theaccretion efficiency in the (cold) accretion disc of AGNs and the physicalstatus of the (hot) corona responsible for the X-ray emission.

The SDSS/XMM-Newton Quasar Survey: Correlation between X-ray spectral slope and Eddington ratio