The MEXSAS2 Sample and the Ensemble X-ray Variability of Quasars

TL;DR

This paper introduces the MEXSAS2 sample, analyzing X-ray variability in quasars, confirming known trends and proposing a new spectral variability estimator with comprehensive parameter dependence analysis.

Contribution

The study presents the second MEXSAS sample and a novel spectral variability estimator, expanding understanding of quasar X-ray variability and its dependence on physical parameters.

Findings

Structure function decreases with X-ray luminosity.

Weak dependence of variability on black hole mass.

Confirmed softer-when-brighter spectral trend.

Abstract

We present the second Multi-Epoch X-ray Serendipitous AGN Sample (MEXSAS2), extracted from the 6th release of the XMM Serendipitous Source Catalogue (XMMSSC-DR6), cross-matched with Sloan Digital Sky Survey quasar catalogues DR7Q and DR12Q. Our sample also includes the available measurements for masses, bolometric luminosities, and Eddington ratios. Analyses of the ensemble structure function and spectral variability are presented, together with their dependences on such parameters. We confirm a decrease of the structure function with the X-ray luminosity, and find a weak dependence on the black hole mass. We introduce a new spectral variability estimator, taking errors on both fluxes and spectral indices into account. We confirm an ensemble softer when brighter trend, with no dependence of such estimator on black hole mass, Eddington ratio, redshift, X-ray and bolometric luminosity.