X-RAY NOVA IN MUSCA (GRS-1124-68) - HARD X-RAY SOURCE WITH NARROW ANNIHILATION LINE

On 1991 January 20-21, the SIGMA telescope on board GRANAT detected a relatively narrow variable emission line near 500 keV with net flux approximately 6 x 10(-3) photons s-1 cm-2 in the spectrum of the X-ray Nova Muscae. During 1991 January 9-August 14, the spectrum had a strong hard component extending up to approximately 300 keV with average power-law photon index approximately 2.5. This component varies by a factor approximately 2 on time scales of hours. The hard component was absent only once, on April 12, but it reappeared during subsequent observations on May 31 and August 14. In contrast to the soft component, the light curve of the hard X-ray component does not decay exponentially, and so its relative contribution to the luminosity increased with time.