Active galactic nuclei.: VI.: ROSAT variability of Seyfert galaxies

X-ray photon trains from Seyfert 1 galaxies by ROSAT contain both nondeterministic and deterministic variable components intrinsic to the source. Statistical procedures utilizing principal component analysis and the wavelet transform method permit elimination of the satellite wobble effect and removal of the apparent-luminosity-correlated variable components, which is mostly due to photon arrival time statistics, thus leaving the small source-related nondeterministic and deterministic components. Some of the deterministic structures seen exhibit quasiperiodic behavior in X-ray variability from between 20 and 200 s. We present a detailed analysis of some of the ROSAT data for NGC 5548, in which we find that these structures usually persist for only between 50 and 200 s, but infrequently they last much longer; in one case an similar to 80 s quasiperiodicity persisted for an entire observation period, about 1900 s. Furthermore, during that period the X-ray power in the range between 0.1 and 2.4 keV carried by the deterministic structures was as much as 17.2% of the total. This intrinsic variability-whether deterministic or nondeterministic-can be modeled by the superposition of discrete luminosity "building blocks." In this paper we suggest that these building blocks are due to short super-Eddington ballistic accretion events resulting from small (similar to 10 M.) black holes passing through accretion disks surrounding the large and intermediate size black holes in a nuclear cluster of compact objects in an AGN. In particular, we show that the luminosity of the deterministic structures in NGC 5548 is within the range of the luminosity such events should produce.