Optical Light Curves 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-rayobservations of the black hole soft X-ray transient GRS 1124-68 in outburst canalso explain the optical lightcurves of a similar object A0620-00. We show thatto reproduce the observed X-ray to optical flux ratio in A0620-00, we need toassume X-ray irradiation of the outer disk that is significantly in excess ofwhat is expected for a standard planar disk. With enhanced irradiation, theEsin et al. model can reproduce the optical evolution of A0620-00 in outburstvery well. Though we find that optical observations of GRS 1124-68 also implyenhanced X-ray heating of the outer disk, the irradiation appears to be afactor 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 maybe due to disk warping, and that the degree of warping differs between the twobinaries.Comment: 12 pages, including 4 figures and a table, LaTeX (emulateapj.sty; newapa.sty for bibliography definitions); accepted for publication in Ap