REVIEW OF 3 YEARS OF OBSERVATIONS WITH THE LOW-ENERGY GAMMA-RAY TELESCOPE-SIGMA ONBOARD GRANAT

The French hard X-ray, soft gamma-ray telescope SIGMA, one of the main instruments onboard the Soviet satellite Granat, has performed sky observations in the 35 keV-1.3 MeV range during the last 3 years, with unprecedented angular resolution (a few arcminutes) in this energy range. New results have been obtained by imaging the Galactic center region with a 1.5' accuracy during more than 800 hr distributed in six periods which took place in early spring and autumn, from 1990 to 1992. The 6-degrees X 6-degrees region centered on the Galactic center contains more than six highly variable sources in the 100 keV domain. Galactic sources, mainly associated with hard X-ray binaries, as well as bright extragalactic objects have been analyzed and monitored during 1990-1992. The main result obtained by SIGMA was demonstrating the variability of the sky in the hard X-ray domain and pointing out the outstanding consequences of the angular resolution on the understanding of high-energy emission sites.