The Hard X-Ray Spectral Slope as an Accretion Rate Indicator in Radio-quiet Active Galactic Nuclei

We present new XMM-Newton observations of two luminous and highaccretion-rate radio-quiet active galactic nuclei (AGNs) at z~2. Together witharchival X-ray and rest-frame optical spectra of three sources with similarproperties as well as 25 moderate-luminosity radio-quiet AGNs at z<0.5, weinvestigate, for the first time, the dependence of the hard (>~2 keV) X-raypower-law photon index on the broad H_beta emission-line width and on theaccretion rate across ~3 orders of magnitude in AGN luminosity. Provided theaccretion rates of the five luminous sources can be estimated by extrapolatingthe well-known broad-line region size-luminosity relation to high luminosities,we find that the photon indices of these sources, while consistent with thoseexpected from their accretion rates, are significantly higher than expectedfrom the widths of their H_beta lines. We argue that, within the limits of oursample, the hard-X-ray photon index depends primarily on the accretion rate.Comment: 4 pages (emulateapj), 2 figures. Accepted for publication in ApJ Letter