On the relation between hard X-ray photon index versus accretion rate for super-Eddington accreting quasars

Abstract

We investigate whether the hard X-ray photon index ($\Gamma$) versus accretion rate correlation 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, including 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 $\mathrm{H}\beta$ lines, and we use the standard thin disk model to derive the dimensionless accretion rates ($\dot{\mathcal{M}}$) 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 $\Gamma$ values. We find a statistically significant ($R_{\mathrm{S}}=0.43$, $p=7.75\times {10}^{-3}$) correlation between $\Gamma$ and $\dot{\mathcal{M}}$ for the super-Eddington subsample. The $\Gamma$-$\dot{\mathcal{M}}$ correlation for the sub-Eddington subsample is also significant, but weaker ($R_{\mathrm{S}}=0.30$, $p=9.98\times {10}^{-3}$). Linear regression analysis shows that $\mathrm{\Gamma }=(0.34\pm 0.11)\log \dot{\mathcal{M}}+(1.71\pm 0.17)$ and $\mathrm{\Gamma }=(0.09\pm 0.04)\log \dot{\mathcal{M}}+(1.93\pm 0.04)$ for the super- and sub-Eddington subsamples, respectively. The $\Gamma$-$\dot{\mathcal{M}}$ 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 $\Gamma$-$\dot{\mathcal{M}}$ 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.

14 pages, 10 figures, accepted for publication in ApJ

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