EXOSAT Observatory, Astrophys. Div., ESA R. Mushotzky Lab. for High Energy Astrophys. GSFC, NASA P. Giommi EXOSAT Observatory, Astrophys. Div. ESA J. Clavel IUE Observatory, Astrophys. Div., ESA W. Wamsteker IUE Observatory, Astrophys. Div., ESA SUMMARY Recent EXOSAT observations of active galactic nuclei are presented. Unlike earlier X-ray satellites (all of which flew in low earth orbit), the deep orbit of EXOSAT allows long continuous observations of celestial X-ray sources, uninterrupted by

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... earth occultation etc. We present the results of EXOSAT observations of several AGN which have been seen to vary rapidly (timescale 0.2-6 hours). We also consider the implications of rapid variability in AGN. For Seyfert galaxies and quasars, we find a highly significant correlation between the timescale of variability and their X-ray luminosity. They are not, howwever, bounded either by the (classical) Eddington limit nor by efficiency arguments. We sugest, rather, that the emitting plasma is dominated by electron-positron pairs. Several AGN have been seen to vary on short timescales. The nearby spiral M81 was found to contain a powerful X-ray source (L x -3 χ 10*° ergs/s) which showed repeated variations by a factor two in ten minutes or so. This is the lowest luminosity for any identified AGN.In three out of five EXOSAT observations (each lasting between five and ten hours), NGC 4593 was seen to vary by a factor two, always on a timescale of two hours or so. The more luminous Seyfert I NGC 7469 varied by a factor two in six hours during one observation (out of three). A six hour observation of the Seyfert I galaxy Fairall-9 in July 1985 revealed repeated rapid fluctuations ba a factor two in 15 minutes or so. The implied efficiency of conversion of matter to energy for this object (Fabian & Rees 1978) is T[%>8% 9 as extreme as for the rapid variability found by Tennant et al. (1981) for the (much lower luminosity object) NGC 6814. What can be learnt from observations of rapid X-ray variability in AGN? We have compiled from the literature an extensive sample of AGN whose reported X-ray variability is reliably established, time resolved and the fastest reported for that particular object. In Fig. 1 we plot the logarithm of the rest-frame 2-10 keV luminosity, log L x , against the time for the source intensity to double, At. The BL Lac objects are considered separately from the Seyferts and 269