The intrinsic X-ray luminosity distribution of an optically-selected SDSS quasar population

TL;DR

This study models the intrinsic X-ray luminosity distribution of optically-selected SDSS quasars, revealing its dependence on UV luminosity and redshift, and providing insights into quasar emission properties.

Contribution

It introduces a hierarchical Bayesian framework to characterize the intrinsic X-ray luminosity distribution of SDSS quasars, accounting for redshift and luminosity dependencies.

Findings

X-ray luminosity distribution is Gaussian in log space.

Mean X-ray luminosity depends on UV luminosity and redshift.

Redshift evolution affects the $L_{2500}$-$L_X$ and $ ext{α}_ ext{ox}$ relations.

Abstract

In active galactic nuclei, the relationship between UV and X-ray luminosity is well studied (often characterised by $\alpha_\text{ox}$) but often with heterogeneous samples. We have parametrized the intrinsic distribution of X-ray luminosity, $L_\text{X}$, for the optically-selected sample of SDSS quasars in the Stripe 82 and XXL fields across redshifts 0.5-3.5. We make use of the available XMM observations and a custom pipeline to produce Bayesian sensitivity curves that are used to derive the intrinsic X-ray distribution in a hierarchical Bayesian framework. We find that the X-ray luminosity distribution is well described by a Gaussian function in ${\log_{10}}L_\text{X}$ space with a mean that is dependent on the monochromatic 2500A UV luminosity, $L_{2500}$. We also observe some redshift dependence of the distribution. The mean of the $L_\text{X}$ distribution increases with redshift while the width decreases. This weak but significant redshift dependence leads to $L_{2500}$-$L_\text{X}$ and $L_{2500}$-$\alpha_\text{ox}$ relations that evolve with redshift, and we produce a redshift- and $L_{2500}$-dependent $\alpha_\text{ox}$ equation. Neither black hole mass nor Eddington ratio appear to be potential drivers of the redshift evolution.