Outbursts in Black Hole X-ray Transients: Clues from Multiwavelength Observations

Abstract

Results from a HST/XTE/multiwavelength campaign to observe X-ray transients in outburst will be described, focussing on what we learned in our extensive observations of the summer 1996 outburst of GRO J1655-40. The talk will begin with an introduction outlining the relevant theoretical models: the disk limit cycle instability and the possibility of advective flows. HST observations of other transients made prior to 1996 will be described. The analysis of our GRO J1655-40 dataset will be discussed in the light of the following issues: The optical/UV spectrum could be interpreted as thermal or possible non- thermal emission: evidence for each interpretation and the consequent conclusions will be presented. There is an apparent anti-correlation between the optical/UV fluxes and the hard X-ray flux during the optical decline. Because the disk in this system is so large, there is no global steady-state solution for the disk with a sub-Eddington mass transfer rate. The UV resonance line profiles are suggestive of a bi-conical disk wind. Independent clues to the structures in the system may arise from echo-mapping of the variations in the 2-12 keV, UV, and optical light curves. These results will be summarised and compared to the predictions of the advective models, the `simple' disk instability model, and to the modifications to the latter arising when the effects of irradiation are included.