Rapid X - ray Variability of Seyfert 1 Galaxies

The rapid and seemingly random fluctuations in X-ray luminosity of Seyfert galaxies provided early support for the standard model in which Seyferts are powered by a supermassive black hole fed from an accretion disc. However, since EXOSAT there has been little opportunity to advance our understanding of the most rapid X-ray variability. Observations with XMM-Newton have changed this. We discuss some recent results obtained from XMM-Newton observations of Seyfert 1 galaxies. Particular attention will be given to the remarkable similarity found between the timing properties of Seyferts and black hole X-ray binaries, including the power spectrum and the cross spectrum (time delays and coherence), and their implications for the physical processes at work in Seyferts. X-ray variability appears to be ubiquitous in Active Galactic Nuclei (AGN). The rapid and seemingly random fluctuations in the X-ray luminosity of Seyfert galaxies provided early support for the standard black hole/accretion disc model (Rees, 1984) by implying compact emission regions and high luminosity densities (Barr & Mushotzky, 1986). 2. The EXOSAT era EXOSAT (1983–1986) was the first mission to provide long (∼ 3 day), uninterrupted X-ray observations of Seyfert galaxies. From these observations the X-ray power spectra (see van der Klis, 1989) of Seyfert galaxies were measured for the first time (Lawrence et al. The EXOSAT observations showed that the power spectra of Seyferts above ∼ 10 −5 Hz could be approximated by a power-law: P(f) ∝ f −α where P(f) is the power at frequency f and α is the power spectrum slope. The measured slopes from the EXOSAT observations were typically α ≈ 1.5. Processes such as these, which have broad-band power spectra with more power at