SIGMA GRANAT SOFT GAMMA-RAY OBSERVATIONS OF THE X-RAY NOVA IN MUSCA - DISCOVERY OF POSITRON-ANNIHILATION EMISSION-LINE

The day after its discovery by the Watch instrument, the X-ray nova GRS 1124-684 in Musca was detected by the soft gamma-ray telescope SIGMA at the limit of its field of view. SIGMA pointed the source seven other times between 1991 January and February, and GRS 1124-684 has always been detected up to 300 keV, showing it was one of the hardest objects of the sky. After the flare of January 9 the average spectrum is well fitted by a power law of index 2.38, and the light curve shows a slower decrease than observed at low energy with superposed variability on time scales of several hours. However, the most remarkable result is the discovery of an emission feature around 500 keV in the source spectrum during one postflare observation. The line was detected in the images at a level of 5.1-sigma; it had a width compatible with the detector spectral resolution and a flux of 6.0 x 10(-3) photons cm-2 s-1. This is first clear evidence of gamma-ray line emission from soft X-ray transients, and, if interpreted as a positron annihilation line, it strongly suggests the X-ray nova in Musca to be a new black hole candidate.