Active galactic nuclei. VI. ROSAT variability of Seyfert galaxies

X-ray photon trains from Seyfert 1 galaxies by ROSAT contain both nondeterm inistic and deterministic variable components intrinsic to the source. Stat istical procedures utilizing principal component analysis and the wavelet t ransform method permit elimination of the satellite wobble effect and remov al of the apparent-luminosity-correlated variable components, which is most ly due to photon arrival time statistics, thus leaving the small source-rel ated nondeterministic and deterministic components. Some of the determinist ic 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 f or only between 50 and 200 s, but infrequently they last much longer; in on e case an similar to 80 s quasiperiodicity persisted for an entire observat ion period, about 1900 s. Furthermore, during that period the X-ray power i n 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 dete rministic or nondeterministic-can be modeled by the superposition of discre te luminosity "building blocks." In this paper we suggest that these buildi ng blocks are due to short super-Eddington ballistic accretion events resul ting from small (similar to 10 M.) black holes passing through accretion di sks surrounding the large and intermediate size black holes in a nuclear cl uster of compact objects in an AGN. In particular, we show that the luminos ity of the deterministic structures in NGC 5548 is within the range of the luminosity such events should produce.