In either case it is accretion onto the compact object which is responsible for the gamma-radiation. A more accurate spectral measurement using OSSE gave support to the black-hole scenario, as did the fact that periodic pulsations - a sign that the radiation is associated with the rotation of a neutron star - were not evident in the data. The true nature of GRO J1655-40 could not be unambiguously determined by Compton alone - observations at radio, optical and x-ray are required to distinguish between the likely possibilities, or perhaps reveal some unexpected alternative.

The radio images of GRO J1655-40. The ejection of "blobs" of material which stream away from the central source can be discerned.

The BATSE time history of gamma-ray emission from the transient source GRO J1655-40 as compared to the radio lightcurve.

Optical studies showed that its orbital period was 2.6 days; 8-12 hours is more typical. However, the real surprise was in the radio. GRO J1655-40 was found to be ejecting matter in highly collimated streams in a direction nearly perpendicular to the plane of its binary orbit. Astronomers call such streams of matter, "jets", in analogy to the stream of water emitted from the nozzle of a high-pressure water hose. Jets had been seen before, and are in fact quite common, in extremely distant, energetic objects

known as "quasars". However, in our own galaxy, there are only a few examples of jet-producing sources. What made GRO J1655-40 all the more interesting was that time-lapsed sequences of observations revealed that the jet- ejection velocities approached the speed of light; 92% of the speed of light to be precise! Einstein's theory of relativity dictates that no material object move with a velocity that exceeds the speed of light, so GRO J1655-40 appeared to

be hurling large quantities of material into space at 92% of the "cosmic speed limit". Actually not quite: scientists studying quasars had noted in the 1970's that a combination of projection effects and relativity can produce the illusion of motion at, or even exceeding the speed of light, but it still remains a hard and fast truth that physical matter never really exceeds the speed of light.

To gamma-ray astronomers, or for that matter to scientists in general, the prospect of finding the unexpected is often the most exciting endeavor! For GRO J1655-40, the team of gamma-ray astronomers using CGRO, radio astronomers using the Very Large Array (VLA - an array of radio telescopes mounted in an 22-mile long "Y" shaped configuration on the desert of New Mexico), and astronomers working in the visible domain using telescopes in the Chilean Andes were not disappointed! What they discovered was that GRO J1655- 40 did in fact contain a black hole.