BROAD-BAND HIGH-ENERGY OBSERVATIONS OF THE SUPERLUMINAL JET SOURCE GRO J1655-40 DURING AN OUTBURST

The X-ray/radio transient superluminal jet source GRO J1655-40 was rec ently suggested to contain a black hole from optical observations. Bec ause it is a relatively close-by system (d similar to 3.2 kpc), it can likely provide us with rich information about the physics operating i n both Galactic and extragalactic jet sources. We present the first si multaneous broadband high-energy observations of GRO J1655-40 during t he 1995 July-August outburst by three instruments: ASCA, WATCH/Granat, and BATSE/CGRO, in the energy band from 1 keV to 2 MeV. Our observati ons strengthen the interpretation that GRO J1655-40 contains a black h ole. We detected a two-component energy spectrum, commonly seen from o ther Galactic black hole binaries, but never detected from a neutron s tar system. Combining our results with the mass limits derived from op tical radial velocity and orbital period measurements, we further cons train the mass of the central object to be between 3.3 and 5.8 M., abo ve the well-established mass upper limit of 3.2 M. for a neutron star (the optical mass function for GRO J1655-40 is 3.16 +/- 0.2 M.). This system is therefore the first Galactic superluminal jet source for whi ch there is strong evidence that the system contains a stellar mass bl ack hole. The inclination angle of the binary system is constrained to be between 76 degrees and 87 degrees, consistent with estimates obtai ned from optical light curves and radio jet kinematics.