Optical Lightcurves of the Black Hole Binaries GRS 1124-68 and A0620-00 in Outburst: the Importance of Irradiation

We test whether the model proposed by Esin et al. to explain X-ray observations of the black hole soft X-ray transient GRS 1124-68 in outburst can also explain the optical lightcurves of a similar object A0620-00. We show that to reproduce the observed X-ray to optical flux ratio in A0620-00, we need to assume X-ray irradiation of the outer disk that is significantly in excess of what is expected for a standard planar disk. With enhanced irradiation, the Esin et al. model can reproduce the optical evolution of A0620-00 in outburst very well. Though we find that optical observations of GRS 1124-68 also imply enhanced X-ray heating of the outer disk, the irradiation appears to be a factor of roughly 3 weaker in this system than in A0620-00. This is surprising, since GRS 1124-68 has a larger disk. We speculate that enhanced irradiation may be due to disk warping, and that the degree of warping differs between the two binaries.