  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, alpha_ox, and the
X-ray photon index, Gamma_X, 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 Gamma_X from Chandra. We
estimate M_BH 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 (10^6 <
M_BH / M_Sun < 10^10) and redshift (z < 4.8). 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 M_BH 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 ~ 3 x 10^8 M_Sun. 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
Eddington. In addition, we argue that the Gamma_X 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 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.
