X‐Ray Fluctuation Power Spectral Densities of Seyfert 1 Galaxies

By combining complementary monitoring observations spanning long, medium andshort time scales, we have constructed power spectral densities (PSDs) of sixSeyfert~1 galaxies. These PSDs span $\gtrsim$4 orders of magnitude in temporalfrequency, sampling variations on time scales ranging from tens of minutes toover a year. In at least four cases, the PSD shows a "break," a significantdeparture from a power law, typically on time scales of order a few days. Thisis similar to the behavior of Galactic X-ray binaries (XRBs), lower masscompact systems with breaks on time scales of seconds. NGC 3783 shows tentativeevidence for a doubly-broken power law, a feature that until now has only beenseen in the (much better-defined) PSDs of low-state XRBs. It is alsointeresting that (when one previously-observed object is added to make a smallsample of seven), an apparently significant correlation is seen between thebreak time scale $T$ and the putative black hole mass $M_{\rm BH}$, while noneis seen between break time scale and luminosity. The data are consistent withthe linear relation $ T = M_{\rm BH}/10^{6.5} \Msun$; extrapolation over 6--7orders of magnitude is in reasonable agreement with XRBs. All of thisstrengthens the case for a physical similarity between Seyfert~1s and XRBs.Comment: 27 pages, 13 figures. Accepted for publication in ApJ. Typo correcte