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 of about 0.006 photons/s/sq cm 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 about 300 keV with average power-law photon index about 2.5. This component varies by a factor about 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.