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, alpha(ox), and the monochromatic optical luminosity, l(opt). 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 alpha(ox) to be defined at arbitrary frequencies, rather than the standard 2500 angstrom and 2 keV. We find that while the choice of optical wavelength does not strongly influence the alpha(ox)-l(opt) relation, the slope of the relation does depend on the choice of X-ray energy. The slope of the relation becomes steeper when alpha(ox) is defined at low (similar to 1 keV) X-ray energies. This change is significant when compared to the slope predicted by a decrease in the baseline over which alpha(ox) is defined. The slopes are also marginally flatter than predicted at high (similar to 10 keV) X-ray energies. Partial correlation tests show that while the primary driver of alpha(ox) is l(opt), the Eddington ratio correlates strongly with alpha(ox) when l(opt) is taken into account, so accretion rate may help explain these results. We combine the alpha(ox)-l(opt) and Gamma-L(bol)/L(Edd) relations to naturally explain two results: (1) the existence of the Gamma-l(x) relation as reported in Young et al., and (2) the lack of a Gamma-l(opt) 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 alpha(ox) and Eddington ratio. This confirms that an increase in accretion rate correlates with a decrease in the fraction of up-scattered disk photons.