EVOLUTION OF THE DOUBLE NEUTRON-STAR MERGING RATE AND THE COSMOLOGICAL ORIGIN OF GAMMA-RAY BURST SOURCES

Evolution of the coalescence rate of double neutron stars (NSs) and ne utron star-black hole (BH) binaries are computed for model galaxies wi th different star formation rates. Assuming gamma-ray bursts (GRBs) to originate from NS + NS or NS + BH merging in distant galaxies, theore tical log N-log S distributions for GRBs are calculated for the first time taking the computed merging rates into account. We use a flat cos mological model (Omega = 1) with different values of the cosmological constant A and under various assumptions about the star formation hist ory in galaxies. The calculated source evolution predicts a 5-10 times increase of the source statistics at count rates 3-10 times lower tha n the existing BATSE sensitivity limit. The most important parameter i n fitting the second BATSE catalog is the initial redshift of star for mation, which is found to be z() = 2-5, depending on a poorly determi ned average spectral index of GRBs.