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Astron. Astrophys. 332, L57-L60 (1998)

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1. Introduction

The recent discoveries of optical counterparts of the [FORMULA] -ray bursts GRB 970228 (Groot et al. 1997) and GRB 970508 (Bond 1997) and the measured emission-line redshift of [FORMULA] (Metzger et al. 1997) for the latter provide evidence for their cosmological origin. A possible source for [FORMULA] -ray bursts (GRBs) may be the merging of two neutron stars in a close binary (Blinnikov et al. 1984). For cosmologies with no vacuum energy, the brightness distribution of burst intensities expected for a uniform source population is consistent with the BATSE distribution if the limiting redshift is about unity (e.g. Dermer 1992; Mao & Paczyski 1992). However, cosmological time-dilation effects in the BATSE sample indicate that the dimmest sources should be located at [FORMULA] (Norris et al. 1995). If this is the actual limiting redshift, a source population with a comoving rate density of the form [FORMULA] is compatible with the BATSE distribution for 1.5 [FORMULA] 2 (Horack et al. 1995). Therefore, it is worthwhile to examine whether the comoving merger rate density of neutron-star binaries evolves in a similar fashion.

By means of population synthesis computations for binary stars the merger rate of neutron-star binaries [hereafter [FORMULA] ] can be computed (see Portegies Zwart & Yungelson 1998 and references therein). Using this approach Lipunov et al. (1995) computed the evolution of [FORMULA] mergers as a function of redshift and [FORMULA] distributions of GRBs for a cosmic population which contains galaxies with a constant star-formation rate and also galaxies with initial bursts of star formation in different proportions. Totani (1997) computed the evolution of the GRBs rate density from [FORMULA] mergers in a model based on the observationally determined history of cosmic star formation and in a model derived from detailed galaxy evolution. Totani assumed a distribution of merging times [FORMULA] for [FORMULA] systems 1. We go a step further by adopting for all galaxies exponentially decreasing star formation rates (SFR) with different timescales depending on galaxy morphology (e. g. Sandage 1986). We apply the distribution of [FORMULA] from model B of Portegies Zwart & Yungelson (1998, henceforth PZY98) which provides the best fit to the expected birthrate and orbital parameters of the Galactic population of high-mass binary pulsars.

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© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998