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.