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