Oxford University Press, Monthly Notices of the Royal Astronomical Society, 2(315), p. 325-336, 2000
DOI: 10.1046/j.1365-8711.2000.03385.x
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We present an analysis of X-ray variability in a flux limited sample of QSOs. Selected from our deep ROSAT survey, these QSOs span a wide range in redshift ($0.10.5$) do not show the anti-correlation 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.