On the Relation between the Hard X-Ray Photon Index and Accretion Rate for Super-Eddington Accreting Quasars

Abstract

We investigate whether the correlation between the hard X-ray photon index ( ) and accretion rate for super-Eddington accreting quasars is different from that for sub-Eddington accreting quasars. We construct a sample of 113 bright quasars from the Sloan Digital Sky Survey Data Release 14 quasar catalog, with 38 quasars as the super-Eddington subsample and 75 quasars as the sub-Eddington subsample. We derive black hole masses using a simple-epoch virial mass formula based on the lines, and we use the standard thin disk model to derive the dimensionless accretion rates ( ) for our sample. The X-ray data for these quasars are collected from the Chandra and XMM-Newton archives. We fit the hard X-ray spectra using a single power-law model to obtain values. We find a statistically significant ( , ) correlation between and for the super-Eddington subsample. The – correlation for the sub-Eddington subsample is also significant, but weaker ( , ). Linear regression analysis shows that and for the super- and sub-Eddington subsamples, respectively. The – correlations of the two subsamples are different, suggesting different disk–corona connections in these two types of systems. We propose one qualitative explanation of the steeper – correlation in the super-Eddington regime that involves larger seed photon fluxes received by the compact coronae from the thick disks in super-Eddington accreting quasars.