Echoes in X-ray Binaries

We present a method of analysing the correlated X-ray and optical/UV variability in X-ray binaries, using the observed time delays between the X-ray driving lightcurves and their reprocessed optical echoes. This allows us to determine the distribution of reprocessing sites within the binary. We model the time-delay transfer functions by simulating the distribution of reprocessing regions, using geometrical and binary parameters. We construct best-fit time-delay transfer functions, showing the regions in the binary responsible for the reprocessing of X-rays. We have applied this model to observations of the Soft X-ray Transient, GROJ1655-40.We find the optical variability lags the X-ray variability with a mean time delay of 19.3±2.2 seconds. This means that the outer regions of the accretion disc are the dominant reprocessing site in this system. On fitting the data to a simple geometric model, we derive a best-fit disk halfopening angle of 13.5 −2.8 degrees, which is similar to that observed after the previous outburst by Orosz & Bailyn (1997). This disk thickening has the effect of almost entirely shielding the companion star from irradiation at this stage of the outburst.