Understanding spectral variability and time lags in accreting black holes Original Research Article

I review the temporal/spectral data of accreting black hole sources paying most attention to the properties of the temporal variability such as photon energy dependent auto- and cross-correlation functions, average shot profiles and hardness ratios, and the Fourier frequency dependent time/phase lags. These statistics characterize spectral changes at short time scales that are otherwise impossible to study by direct spectral analysis. The data provide strong constraints on the theoretical models for X-ray production in accreting black holes. Models for the spectral variability of the Comptonized component are briefly reviewed. It is also shown that Compton reflection can have significant impact on the observed temporal characteristics.