GAMMA-RAY BURSTS AND DENSITY EVOLUTION OF NEUTRON-STAR BINARY MERGERS

The evolution of the comoving cosmic merger-rate density of neutron st ar binaries n(c)(z) is calculated using a distribution of their mergin g times provided by population-synthesis computations of binary stars. We adopt an exponential law for the star formation rate with various timescales for different morphological types of galaxies. For elliptic al galaxies also an initial burst of star formation, lasting one Gyr, is considered. The resulting n(c)(z) of most models agree with the for m n(c)(z) proportional to (1 + z)(1.5-2) for z less than or similar to 2, which has been proposed for the source population of gamma-ray bur sts. Assuming a standard candle luminosity, the computed brightness di stribution is consistent with the BATSE results if bursts at the peak flux threshold, P = 0.4 photons cm(-2) s(-1), are located at a limitin g redshift of 1.9 to 3.3. Progenitors of the systems producing gamma-r ay bursts at small redshift (bright) are likely to host in spiral gala xies and star forming regions whereas these at high redshift (dim) res ide mainly in elliptical galaxies. The location of a burst may be up t o similar to 1 MPc away from the host galaxy.