Modeling the Time Variability of Black Hole Candidates

We present model light curves, Power Spectral Densities (PSD) and time lags of accreting Black Hole Candidates (BHC) based on a recent model of these sources. According to our model the observed variability is due, to a large extent, to the stochastic nature of Comptonization, the basic process responsible also for the emission of high energy radiation. Our additional assumption is that the Comptonization process takes place in an extended but non-uniform atmosphere around the compact object. Our model reproduces the observed light curves well, in that it provides a good fit to the PSDs and the overall light curve morphology, indicating, in accordance with observation, that most of the power resides at time scales $\gsim$ a few seconds while at the same time one can distinguish shots of a few msec in duration. We suggest that refinement of this type of model along with spectral and phase lag information can be used to probe the structure of this class of high energy sources.