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 neutron star-black hole (BH) binaries are computed for model galaxies with different star formation rates. Assuming gamma-ray bursts (GRBs) to originate from NS + NS or NS + BH merging in distant galaxies, theoretical log N-log S distributions for GRBs are calculated for the first time taking the computed merging rates into account. We use a flat cosmological model (Omega = 1) with different values of the cosmological constant A and under various assumptions about the star formation history in galaxies. The calculated source evolution predicts a 5-10 times increase of the source statistics at count rates 3-10 times lower than the existing BATSE sensitivity limit. The most important parameter in fitting the second BATSE catalog is the initial redshift of star formation, which is found to be z(*) = 2-5, depending on a poorly determined average spectral index of GRBs.