Does the optical-to-X-ray energy distribution of quasars depend on optical luminosity?

Abstract

We report on a detailed analysis of the correlation between the optical-UV (Lo) and X-ray (Lx) luminosities of quasars by means of Monte Carlo simulations, using a realistic luminosity function. We find, for a quasar population with an intrinsically constant, mean X-ray loudness alpha_ox, that the simulated alpha_ox - Lo relation can exhibit various `apparent' properties, including an increasing alpha_ox with Lo, similar to what has been found from observations. The determining factor for this behavior turns out to be the relative strength of the dispersions of the luminosities, i.e. their deviations from the mean spectral energy distribution at the optical and X-ray bands, such that a dispersion larger for the optical luminosity than for the X-ray luminosity tends to result in an apparent correlation. We suggest that the observed alpha_ox - Lo correlation can be attributed, at least to some extent, to such an effect, and is thus not an underlying physical property. The consequences of taking into account the luminosity dispersions in an analysis of the observed luminosity correlations is briefly discussed. We note that similar considerations might also apply for the Baldwin effect.