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 variabil ity observed from this region during balloon flights can be explained by the variability of several point sources scattered over several deg rees region near Galactic Center. At energies higher than approximatel y 60 keV the contribution from two sources, 1E 1740.7-2942 and GRS 175 8-258, dominates. On the basis of hard spectra resembling that of Cygn us X-1, these two sources have been added to the list of black hole ca ndidates. On two occasions broad high-energy excesses (200-500 keV) pr obably related to electron-positron annihilation process have been det ected in the spectrum of 1E 1740.7-2942. However, no significant evide nce for a strong flux of narrow 511 keV line emission from any point s ource in the region has been found.