X-Ray, Optical, and Infrared Luminosity Correlations in Radio-quiet Quasars

Abstract

For a sample of 88 radio-quiet quasars that includes 15 X-ray upper limits, I find that the X-ray and IR luminosities follow a significantly nonlinear relation (L_2keV_ ~ L^0.73^_1 micron_) that is consistent with that found for the X-ray and optical luminosities. The redshift dependence is weak and may be nonexistent. The shape of the optical through infrared continuum varies only slightly with luminosity. The most significant change is the lack of a strong ultraviolet bump in low luminosity objects with L_1 micron_ < 10^30^ ergs s^-1^ Hz^-1^. More luminous quasars have significantly stronger bumps, but above 10^30^ ergs s^-1^ Hz^-1^ the strength of the bump does not correlate with luminosity. The relatively constant shape of the infrared through optical continuum implies that a systematic change in the X-ray continuum of quasars with increasing luminosity produces the nonlinear relation between the X-ray luminosity and the optical and IR luminosities. Either the overall normalization of the X-ray continuum relative to the optical and IR must decrease, or the X-ray continuum must flatten with increasing luminosity, or the strength of a soft X-ray excess must decrease with increasing luminosity. Without detailed knowledge of the X-ray and infrared spectra as a function of luminosity, it is difficult to place significant constraints on emission mechanisms.