Broadband 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 recently suggested to contain a black hole from optical observations. Because it is a relatively close-by system (d ~ 3.2 kpc), it can likely provide us with rich information about the physics operating in both Galactic and extragalactic jet sources. We present the first simultaneous broadband high-energy observations of GRO J1655-40 during the 1995 July-August outburst by three instruments: ASCA, WATCH/Granat, and BATSE/CGRO, in the energy band from 1 keV to 2 MeV. Our observations strengthen the interpretation that GRO J1655-40 contains a black hole. We detected a two-component energy spectrum, commonly seen from other Galactic black hole binaries, but never detected from a neutron star system. Combining our results with the mass limits derived from optical radial velocity and orbital period measurements, we further constrain the mass of the central object to be between 3.3 and 5.8 M☉, above 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 which there is strong evidence that the system contains a stellar mass black hole. The inclination angle of the binary system is constrained to be between 76° and 87°, consistent with estimates obtained from optical light curves and radio jet kinematics.