Multi-timescale X-ray reverberation mapping of accreting black holes

Accreting black holes show reflection features in their X-ray spectrum, including an iron Kα line, which result from reprocessing of hard X-ray continuum photons illuminating the accretion disk. Measuring the reverberation lag caused by the differences in path length between direct and reflected emission, and the spectral distortions to the iron line caused by rapid orbital motion and gravitational redshift provides a powerful tool to probe the innermost regions around the black hole. Previous reverberation studies have largely ignored spectral variability of the illuminating continuum. We studied the effect of a pivoting continuum power-law which causes non-linear changes in the shape of the reflection spectrum, and developed an analytic description of the complex cross spectrum as a function of both energy and frequency. I will present our application of this new reverberation model to RXTE data from the black hole X-ray binary Cygnus X-1. We are able to jointly fit the time-averaged X-ray spectrum and the real and imaginary parts of the cross spectrum as a function of energy for a range of Fourier frequencies, thus utilising vastly more information than traditional spectral fitting. I will demonstrate we need to properly account for the continuum variability in order to correctly characterise the parameters of the system such as mass of the central object and inner radius and inclination of the accretion disc.