Observational Constraints on the Dependence of Radio-Quiet Quasar X-Ray Emission on Black Hole Mass and Accretion Rate

In this work we use a sample of 318 radio-quiet quasars (RQQs) to investigate the dependence of the ratio of optical/UV flux to X-ray flux, αox, and the X-ray photon index, ΓX, on black hole mass, UV relative to Eddington luminosity, and X-ray relative to Eddington luminosity. Our sample is drawn from the literature, with X-ray data from ROSAT and Chandra, and optical data mostly from the SDSS; 153 of these sources have estimates of ΓX from Chandra. We estimate MBH using standard estimates derived from the Hβ, Mg II, and C IV broad emission lines. Our sample spans a broad range in black hole mass ( -->106 MBH/M☉ 1010), redshift ( -->0 (1043 λ Lλ(2500 A) [ ergs s−1] 1048) . We find that αox increases with increasing MBH and LUV/LEdd and decreases with increasing LX/LEdd. In addition, we confirm the correlation seen in previous studies between ΓX and MBH and both LUV/LEdd and LX/LEdd; however, we also find evidence that the dependence of ΓX of these quantities is not monotonic, changing sign at -->MBH ~ 3 × 108 M☉. We argue that the αox correlations imply that the fraction of bolometric luminosity emitted by the accretion disk, as compared to the corona, increases with increasing accretion rate relative to the Eddington rate, $d{m}$ -->. In addition, we argue that the ΓX trends are caused by a dependence of X-ray spectral index on -->. We discuss our results within the context of accretion models with Comptonizing corona and discuss the implications of the αox correlations for quasar feedback. To date, this is the largest study of the dependence of RQQ X-ray parameters on black hole mass and related quantities, and the first to attempt to correct for the large statistical uncertainty in the broad-line mass estimates.