  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.
