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

Abstract

In active galactic nuclei, the relationship between UV and X-ray luminosity is well studied (often characterised by α_ox) but often with heterogeneous samples. We have parametrized the intrinsic distribution of X-ray luminosity, L_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 log₁₀ L_X space with a mean that is dependent on the monochromatic 2500 Å UV luminosity, L₂₅₀₀. We also observe some redshift dependence of the distribution. The mean of the L_X distribution increases with redshift while the width decreases. This weak but significant redshift dependence leads to L₂₅₀₀-L_X and L₂₅₀₀-α_ox relations that evolve with redshift, and we produce a redshift- and L₂₅₀₀-dependent α_ox equation. Neither black hole mass nor Eddington ratio appear to be potential drivers of the redshift evolution.