The X-ray/radio transient superluminal jet source GRO J1655-40 was recently suggested to contain a black hole from optical observations .
Being a relatively close-by system ( d \sim 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 broad band 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 _ { \sun } , above the well-established mass upper limit of 3.2 M _ { \sun } for a neutron star ( the optical mass function for GRO J1655-40 is 3.16 \pm 0.2 M _ { \sun } ) .
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 degrees , consistent with estimates obtained from optical light curves and radio jet kinematics .