X-rays from Compton scattering around accreting black holes

Abstract

Compton scattering is one of the most important radiation processes from accreting compact objects. Hard X-rays can be produced by Compton upscattering of soft seed photons from the accretion disk. Here we use both theoretical and observational methods to investigate the hard X-ray emission around accreting Black Holes. We attempt to observationally constrain the geometry of the accretion flow using the observed spectra from the outburst of a transient black hole. The rapid rise of the hard X-ray flux is almost certainly associated with most of the disk mass moving inwards, so the optically thick disk should not extend down to the last stable orbit until the outburst peak. The low/hard state is seen at the start of the rise showing that this is probably associated with a truncated disk. Attempts to derive the inner disk radius from reflection model parameters are inconclusive due to the effects of ionization. We describe a new approach to numerically modelling Compton scattering around accreting sources by solving the distribution functions. This approach (based on work by Guilbert 1981) involves breaking the scattering into individual segments of space and time. We show how a coarse angle grid can be adapted to mimic an arbitrarily fine grid with very little increase in run-time. The resulting code automatically includes time dependent behaviour. We extend the code to calculate the time dependent, self consistent electron distribution resulting from the Compton cooling. This can be used even where the Compton cooling time is shorter than the light crossing time. We show that any system in which the seed photons are dominated by reprocessing should produce soft lags of the order of the light crossing time. Future observations, with more sophisticated satellites, may be able to identify this lag.