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 $H\beta$ lines, and we use
the standard thin disk model to derive the dimensionless accretion rates
($\mathscrM$) 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 ($R_S=0.43$, $p=7.7510^-3$)
correlation between $\Gamma$ and $\mathscrM$ for the super-Eddington
subsample. The $\Gamma$-$\mathscrM$ correlation for the sub-Eddington
subsample is also significant, but weaker ($R_S=0.30$,
$p=9.9810^-3$). Linear regression analysis shows that $\rm
\Gamma=(0.34\pm0.11)log\mathscrM+(1.71\pm0.17)$ and $\rm
\Gamma=(0.09\pm0.04)log\mathscrM+(1.93\pm0.04)$ for the super-
and sub-Eddington subsamples, respectively. The $\Gamma$-$\mathscrM$
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 $\Gamma$-$\mathscrM$
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.
Users
Please
log in to take part in the discussion (add own reviews or comments).