X-ray variability in Black Hole Systems
AGN and Galactic black hole systems are variable in every observable wave band. The X-ray flux exhibits variability on time scales shorter than any other energy band, indicating that the emission occurs in the innermost regions of the central engine. Therefore, a study of the X-ray variability provides an additional powerful tool to probe the extreme physical processes operating in the inner parts of the accretion flow close to the accreting black hole. One of the reasons is that the light curves have been often considered as a by-product of the spectral analysis, which still catalyzes most of the attention and efforts and indeed provides important results. However, often time-averaged X-ray spectra can be equally well described by quite different physical models. The constraints from the temporal and spectral variability can be used to break this spectral degeneracy.In my research, I try to exploit more efficiently the information provided by X-ray data, by combining the standard time-averaged spectral analysis with the investigation of temporal and spectral variability properties in a model-independent way. To this end, I utilize not only linear methods (e.g., PSD, CCF, SF) but also non-linear timing techniques. Non-linear methods are rarely employed in the analysis of AGN light curves, partly because these methods are less developed than linear ones, partly because it is implicitly assumed that AGN light curves are linear and stochastic. However, non-linear methods can be useful not only to characterize the nature of chaotic deterministic systems, but also to discriminate between two time series, regardless if they are linear or non-linear.