REVIEW OF GRANAT SIGMA-OBSERVATIONS OF THE GALACTIC-CENTER REGION

High angular resolution images of the Galactic center region have been obtained in hard X-rays by the GRANAT/SIGMA over 1990-1992. From the first observations it became clear that there is no strong (AGN-like) source at the dynamic center of our Galaxy. Most of the X-ray variability observed from this region during balloon flights can be explained by the variability of several point sources scattered over several degrees region near Galactic Center. At energies higher than approximately 60 keV the contribution from two sources, 1E 1740.7-2942 and GRS 1758-258, dominates. On the basis of hard spectra resembling that of Cygnus X-1, these two sources have been added to the list of black hole candidates. On two occasions broad high-energy excesses (200-500 keV) probably related to electron-positron annihilation process have been detected in the spectrum of 1E 1740.7-2942. However, no significant evidence for a strong flux of narrow 511 keV line emission from any point source in the region has been found.