Ensemble quasar spectral variability from the XMM-Newton Serendipitous
  Source Catalogue
  Variations of the X-ray spectral slope have been found in many Active
Galactic Nuclei (AGN) at moderate luminosities and redshifts, typically showing
a "softer when brighter" behaviour. However, similar studies are not usually
performed for high-luminosity AGNs. We present an analysis of the spectral
variability based on a large sample of quasars in wide intervals of luminosity
and redshift, measured at several different epochs, extracted from the fifth
release of the XMM Newton Serendipitous Source Catalogue. Our analysis confirms
a "softer when brighter" trend also for our sample, extending to high
luminosity and redshift the general behaviour previously found. These results
can be understood in light of current spectral models, such as intrinsic
variations of the X-ray primary radiation, or superposition with a constant
reflection component.
