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 light curves 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-fitting 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 GRO J1655-40. We find that the optical variability lags the X-ray variability with a mean time delay of 19.3 ± 2.2s. 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-fitting disc half-opening angle of 13.5+2.1○-2.8 which is similar to that observed after the previous outburst by Orosz & Bailyn. This disc thickening has the effect of almost entirely shielding the companion star from irradiation at this stage of the outburst.
Publication Source (Journal or Book title)
Monthly Notices of the Royal Astronomical Society
O'Brien, K., Horne, K., Hynes, R., Chen, W., Haswell, C., & Still, M. (2002). Echoes in X-ray binaries. Monthly Notices of the Royal Astronomical Society, 334 (2), 426-434. https://doi.org/10.1046/j.1365-8711.2002.05530.x