We demonstrate correlated rapid variability between the optical/UV and X-ray emission for the first time in a soft X-ray transient, GRO J1655-40: Hubble Space Telescope (HST) light curves show similar features to those seen by the Rossi X-ray Timing Explorer (RXTE), but with mean delay of up to 10-20 s. We interpret the correlations as the result of reprocessing of X-rays into optical and UV emission, with a delay owing to finite light travel time, and thus perform echo mapping of the system. The time-delay distribution has a mean of 14.6 ± 1.4 s and dispersion (i.e. the standard deviation of the distribution) of 10.5 ± 1.9 s at binary phase 0.4. Hence we identify the reprocessing region as the accretion disc rather than the mass donor star.
Publication Source (Journal or Book title)
Monthly Notices of the Royal Astronomical Society
Hynes, R., O'Brien, K., Horne, K., Chen, W., & Haswell, C. (1998). Echoes from an irradiated disc in GRO J1655-40. Monthly Notices of the Royal Astronomical Society, 299 (4) https://doi.org/10.1046/j.1365-8711.1998.01856.x