Observations of cosmic gamma-ray bursts with the main detector of the SIGMA telescope onboard the Granat Observatory

We present the observations of cosmic gamma-ray bursts (GRBs) with the main detector of the SIGMA telescope onboard the Granat Observatory from Januar y 1990 through September 1994. The observations were carried out in the ene rgy range 35-1300 keV. We detected 36 GRBs and 31 high-energy solar flares during this period. No GRB fell within the main field of view; they were al l recorded by the "secondary optics" of the telescope. The SIGMA telescope recorded relatively bright bursts with peak fluxes of 10(-6)-10(-4) erg s(- 1) cm(-2) in the 100-500-keV energy band. Stable detector background allows the long-term variability of GRB sources on a time scale of similar to 100 0 s to be studied. The results of our search for early afterglows of GRBs a re presented. The flux averaged over all bursts in the interval 100-800 s a fter the main event is 0.36 +/- 0.14 counts s(-1)(35-300 keV), suggesting t hat there is soft gamma-ray emission on this time scale after a considerabl e number of GRBs. (C) 2000 MAIK "Nauka/Interperiodica".