Abstract

The time-variable emission from quasars provided some of the earliest evidence that the central engines of these objects were extremely compact, based on simple light-crossing time arguments. Since then, variability has been studied extensively in accreting black hole systems across the mass scale, from X-ray binary systems (XRBs) to Active Galactic Nuclei (AGN), but its physical interpretation has proved difficult. Recent years have seen a breakthrough in our understanding of black hole variability, thanks to the combined study of XRB and AGN variability, new multiwavelength monitoring campaigns and most recently the combination of X-ray spectral and timing information using new analysis techniques. I will describe these developments to show that the flux variability is driven by variations in the mass accretion flow on to the black hole. The fastest variations in both XRBs and AGN show the imprint of X-ray reverberation, due to the light travel-time from the continuum emitting region to the inner accretion disc, which reflects and reprocesses the continuum. I will show how we can use the information contained in the variable light curves to map the emitting regions closest to the black hole.