2003ApJ...593...96M

By combining complementary monitoring observations spanning long, medium, and short timescales, we have constructed power spectral densities (PSDs) of six Seyfert 1 galaxies. These PSDs span ≳4 orders of magnitude in temporal frequency, sampling variations on timescales ranging from tens of minutes to over a year. In at least four cases, the PSD shows a ``break'', a significant departure from a power law, typically on timescales of the order of a few days. This is similar to the behavior of Galactic X-ray binaries (XRBs), lower mass compact systems with breaks on timescales of seconds. NGC 3783 shows tentative evidence for a doubly broken power law, a feature that until now has been seen only in the (much better defined) PSDs of low-state XRBs. It is also interesting that (when one previously observed object is added to make a small sample of seven) an apparently significant correlation is seen between the break timescale T and the putative black hole mass M_BH, while none is seen between break timescale and luminosity. The data are consistent with the linear relation T=M_BH/10^6.5M_☉; extrapolation over 6-7 orders of magnitude is in reasonable agreement with XRBs. All of this strengthens the case for a physical similarity between Seyfert 1 galaxies and XRBs.