SIMBAD references

We present an analysis of X-ray variability in a flux-limited sample of quasi-stellar objects (QSOs). Selected from our deep ROSAT survey, these QSOs span a wide range in redshift (0.1<z<3.2) and are typically very faint, so we have developed a method to constrain the amplitude of variability in ensembles of low signal-to-noise ratio light curves. We find evidence for trends in this variability amplitude with both redshift and luminosity. The mean variability amplitude declines sharply with luminosity, as seen in local active galactic nuclei (AGN), but with some suggestion of an upturn for the most powerful sources. We find tentative evidence that this is caused by redshift evolution, since the high-redshift QSOs (z>0.5) do not show the anticorrelation with luminosity seen in local AGN. We speculate on the implications of these results for physical models of AGN and their evolution. Finally, we find evidence for X-ray variability in an object classified as a narrow-emission-line galaxy, suggesting the presence of an AGN.