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

TL;DR

This study uses simulations to show that the observed correlation between optical and X-ray luminosities in quasars may be largely due to luminosity dispersions rather than an intrinsic physical relationship.

Contribution

The paper demonstrates through Monte Carlo simulations that apparent correlations in quasar luminosities can arise from dispersions, challenging the assumption of an intrinsic optical-X-ray connection.

Findings

Simulated alpha_ox - Lo relation can mimic observed correlations.

Dispersion differences influence apparent luminosity relationships.

Observed correlations may not reflect intrinsic physical properties.

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.