Abstract
Active galactic nuclei (AGN) have been observed to vary in all wavebands on timescales ranging from years to minutes1–3. The timescale of variability can yield information about the efficiency of the emission mechanism4 and the details of the accretion process5. Most of the luminosity of Seyfert galaxies and quasars probably occurs in the hard X-ray region, with a cutoff near 1 MeV (ref. 6). (Although there have been reports of high luminosity ‘ultraviolet bumps’, optical observations of a large sample of objects have shown these to be rare7,8.) We show here that there is a significant correlation between the X-ray luminosity and timescale of X-ray variability for Seyfert galaxies and quasars. We interpret this as evidence that the emitting plasma is near the limit of being dominated by electron-positron pairs. BL Lac objects do not follow this pattern; this may be due either to relativistic beaming or to the differing importance of the pair production process.
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