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, alpha(ox), and the X-ray photon index, Gamma(X), 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 Gamma(X) from Chandra. We estimate M-BH using standard estimates derived from the H beta, Mg II, and C IV broad emission lines. Our sample spans a broad range in black hole mass (10(6) less than or similar to M-BH/M-circle dot less than or similar to 10(10)), redshift (0 < z < 4: 8), and luminosity (10(43) less than or similar to lambda L-lambda(2500 angstrom) [ergs s(-1)]less than or similar to 10(48)). We find that alpha(ox) increases with increasing M-BH and L-UV/L-Edd and decreases with increasing L-X/L-Edd. In addition, we confirm the correlation seen in previous studies between Gamma(X) and MBH and both L-UV/L-Edd and L-X/L-Edd; however, we also find evidence that the dependence of Gamma(X) of these quantities is not monotonic, changing sign at M-BH similar to 3 x 10(8) M-circle dot. We argue that the alpha(ox) 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, m. In addition, we argue that the Gamma(X) trends are caused by a dependence of X-ray spectral index on m. We discuss our results within the context of accretion models with Comptonizing corona and discuss the implications of the alpha(ox) 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.