Ensemble spectral variability study of Active Galactic Nuclei from the
  XMM-Newton serendipitous source catalogue
  The variability of the X-ray spectra of active galactic nuclei (AGN) usually
includes a change of the spectral slope. This has been investigated for a small
sample of local AGNs by Sobolewska and Papadakis, who found that slope
variations are well correlated with flux variations, and that spectra are
typically steeper in the bright phase (softer when brighter behaviour). Not
much information is available for the spectral variability of high-luminosity
AGNs and quasars. In order to investigate this phenomenon, we use data from the
XMM-Newton Serendipitous Source Catalogue, Data Release 5, which contains X-ray
observations for a large number of active galactic nuclei in a wide luminosity
and redshift range, for several different epochs. This allows to perform an
ensemble analysis of the spectral variability for a large sample of quasars. We
quantify the spectral variability through the spectral variability parameter
$\beta$, defined as the ratio between the change in spectral slope and the
corresponding logarithmic flux variation. We find that the spectral variability
of quasars has a softer when brighter behaviour, similarly to local AGNs.
