On the nature of rapid X-ray variability in active galactic nuclei

We systematically analyse all (110) EXOSAT AGN medium-energy light curves of length greater than 20 ks to search for and quantify their flux variability. We also perform extensive simulations to quantify the selection effects (such as the source brightness or the length of observation) that might prevent the detection of variability. We quantify variability in terms of a normalized variability amplitude (NVA) derived from the power spectrum of each light curve. Of the 32 sources observed, we detect variability in 12. We find that, contrary to some previous claims, there is no evidence that sources change the character of their variability between observations. We confirm that there is a strong inverse correlation between NVA and source luminosity which can be simply explained if source size scales with luminosity