SIMBAD references

We present an analysis of X-ray variability in a sample of 156 radio-quiet quasars taken from the ROSAT archive, covering a redshift range 0.1 < z < 4.1. A maximum-likelihood method is used to constrain the amplitude of variability in these low signal-to-noise light curves. Through combining these data in ensembles we are able to identify trends in variability amplitude with luminosity and with redshift. The decline in variability amplitude with luminosity identified in local AGNs (z<0.1) is confirmed out to z=2. There is tentative evidence for an increase in QSO X-ray variability amplitude towards high redshifts (z>2) in the sense that QSOs of the same X-ray luminosity are more variable at z>2 We discuss possible explanations for this effect. The simplest explanation may be that high-redshift QSOs are accreting at a larger fraction of the Eddington limit than local AGNs.