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, α_ox, and the X-ray photon index, Γ_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 Γ_X from Chandra. We estimate M_BH 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 (10⁶lesssim M_BH/M_⊙lesssim 10¹⁰), redshift (0 < z < 4.8), and luminosity (10⁴³lesssim λ L_λ(2500 Å) [ ergs s^-1] lesssim 10⁴⁸) . We find that α_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 Γ_X and M_BH and both L_UV/L_Edd and L_X/L_Edd; however, we also find evidence that the dependence of Γ_X of these quantities is not monotonic, changing sign at M_BH ~ 3 × 10⁸ M_⊙. We argue that the α_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, \dot{m}. In addition, we argue that the Γ_X trends are caused by a dependence of X-ray spectral index on dot m. We discuss our results within the context of accretion models with Comptonizing corona and discuss the implications of the α_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.