Observational Constraints on the Dependence of Radio-Quiet Quasar X-ray Emission on Black Hole Mass and Accretion Rate
Abstract
In this work we use a sample of 318 radio-quiet quasars (RQQ) to investigate the dependence of the ratio of optical/UV flux to X-ray flux, alphaox, and the X-ray photon index, GammaX, on black hole mass, UV luminosity relative to Eddington, and X-ray luminosity relative to Eddington. Our sample is drawn from the SDSS, with X-ray data from ROSAT and Chandra, and optical data mostly from the SDSS; 153 of these sources have estimates of GammaX from Chandra. We estimate MBH using standard estimates derived from the Hbeta, Mg II, and C IV broad emission lines. Our sample spans a broad range in black hole mass (106 < MBH / MSun < 1010) and redshift (z < 4.8). We find that alphaox increases with increasing MBH and LUV / LEdd, and decreases with increasing LX / LEdd. In addition, we confirm the correlation seen in previous studies between GammaX and MBH and both LUV / LEdd and LX / LEdd; however, we also find evidence that the dependence of GammaX of these quantities is not monotonic, changing sign at MBH ~ 3 x 108 MSun. We argue that the alphaox 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 Eddington. In addition, we argue that the GammaX trends are caused by a dependence of X-ray spectral index on accretion rate. We discuss our results within the context of accretion models with comptonizing corona, and discuss the implications of the alphaox 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.