Active Galactic Nuclei. VII. The Spacecraft Wobble and X-Ray Variability   of Seyfert Galaxy NGC 5548
  In discussing the short-time variability of extragalactic X-ray sources, we focus on explaining how wavelet transform in conjunction with non-linear filtering methods are used to remove Poisson statistics and wobble-related variability from the data. This enables the resolution of their intrinsic stochastic, quasi-periodic and deterministic variable components, as well the determination of their persistence. As detailed examples we review and extend the application of these techniques to ROSAT data for the Seyfert I galaxy NGC 5548 and for the QSO 3C273. Besides illustrating the discriminating power of these methods, these treatments confirm the intrinsic character of the transient quasi-periodic and deterministic events in NGC 5548 and 3C273 and enable us to compare both the elementary events and the short-time variability in the two types of sources. These methods can be applied in the investigation of intrinsic source variability in other spectral regions. 
Ludwik Liszka, A. G. Pacholczyk and William R. Stoeger S.J