A HIGH-SENSITIVITY SEARCH FOR GAMMA-RAY BURSTS BY THE SIGMA TELESCOPE ON BOARD GRANAT (THE GALACTIC-CENTER AND ALL SKY DATA)

During more than 2 years of the GRANA T mission the gamma-ray burst (GRB) cell of the narrow aperture coded mask telescope SIGMA was operating for approximately 5300 hr. The coding system of SIGMA is capable to localize with arcmin accuracy the GRB events detected through the 18.1-degrees by 16.8-degrees coded yield of view. The sensitivity of the cell to the on-axis events is very high, approximately 3 x 10(-8) and 8 x 10(-8) ergs cm-2 in the 40-90 keV band for 0.25 and 2.0 s of integration time. Nevertheless, no bursts were actually found within the coded field of view. All 18 cosmic GRB candidates detected by SIGMA till now came from the lateral directions i.e., through sidelobes of the sensitivity diagram, having sensitivity at the level of 10% of the on-axis value, but filling a much larger solid angle, than the coded field of view. For about 950 hr of 5300 the SIGMA was pointed toward the Galactic center (GC), and more than 1.5 x 10(10) M. of Galactic matter was each time within the telescope-coded FOV. For the object, placed at 8.5 kpc distance, the threshold burst luminosity in the 40-90 keV band is 3 x 10(38) ergs s-1, only slightly exceeding the Eddington limit for the neutron star. The absence of weak GRB events from the GC direction indicates that either the luminosity of typical GRBs is below approximately 10(38) ergs s-1 and GRBs are of local origin, or alternatively luminosity is high approximately 10(40) ergs s-1 or greater and most GRBs come from the massive Galactic ''halo'' or are of cosmological origin. Observations of massive extragalactic objects set an upper limit on the frequency of luminous (approximately 10(40)-10(50) ergs s-1) GRB events from these objects. The lack of the weak GRB events observed by SIGMA from all directions is hardly consistent with extrapolation of the KONUS data assuming the Euclidean value of 3/2 for the integral GRB counts slope.