THE X-RAY ENERGY DEPENDENCE OF THE RELATION BETWEEN OPTICAL AND X-RAY EMISSION IN QUASARS

We develop a new approach to the well-studied anti-correlation between the optical-to-X-ray spectral index, αox, and the monochromatic optical luminosity, lopt. By cross-correlating the Sloan Digital Sky Survey DR5 quasar catalog with the XMM-Newton archive, we create a sample of 327 quasars with X-ray signal-to-noise ratio (S/N) > 6, where both optical and X-ray spectra are available. This allows αox to be defined at arbitrary frequencies, rather than the standard 2500 Å and 2 keV. We find that while the choice of optical wavelength does not strongly influence the αox–lopt relation, the slope of the relation does depend on the choice of X-ray energy. The slope of the relation becomes steeper when αox is defined at low (∼1 keV) X-ray energies. This change is significant when compared to the slope predicted by a decrease in the baseline over which αox is defined. The slopes are also marginally flatter than predicted at high (∼10 keV) X-ray energies. Partial correlation tests show that while the primary driver of αox is lopt, the Eddington ratio correlates strongly with αox when lopt is taken into account, so accretion rate may help explain these results. We combine the αox–lopt and Γ–Lbol/LEdd relations to naturally explain two results: (1) the existence of the Γ–lx relation as reported in Young et al., and (2) the lack of a Γ–lopt relation. The consistency of the optical/X-ray correlations establishes a more complete framework for understanding the relation between quasar emission mechanisms. We also discuss two correlations with the hard X-ray bolometric correction, which we show correlates with both αox and Eddington ratio. This confirms that an increase in accretion rate correlates with a decrease in the fraction of up-scattered disk photons.