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

Letter to the Editor

Gamma-ray bursts and density evolution of neutron star binary mergers

Philippe Bagot ¹, Simon F. Portegies Zwart ² and Lev R. Yungelson <sup>3, 4

1</sup> Graal, Upresa 5024/CNRS, Université Montpellier II, CC072, F-34095 Montpellier, France (e-mail: bagot@graal.univ-montp2.fr)
² Astronomical Institute `Anton Pannekoek', Kruislaan 403, 1098 SJ Amsterdam, The Netherlands (e-mail: spz@astro.uva.nl)
³ Institute of Astronomy of the Russian Academy of Sciences, 48 Pyatnitskaya Str., 109017 Moscow, Russia (e-mail: lry@inasan.rssi.ru)
⁴ DARC, Observatoire de Paris, Section de Meudon, F-92190 Meudon Cedex, France

Received 28 October 1997 / Accepted 13 January 1998

Abstract

The evolution of the comoving cosmic merger-rate density of neutron star binaries is calculated using a distribution of their merging 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 elliptical galaxies also an initial burst of star formation, lasting one Gyr, is considered. The resulting of most models agree with the form for z 2, which has been proposed for the source population of -ray bursts. Assuming a standard candle luminosity, the computed brightness distribution 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 limiting redshift of 1.9 to 3.3. Progenitors of the systems producing -ray bursts at small redshift (bright) are likely to host in spiral galaxies and star forming regions whereas these at high redshift (dim) reside mainly in elliptical galaxies. The location of a burst may be up to Mpc away from the host galaxy.

Key words: stars: binaries stars: evolution stars: neutron cosmology: theory gamma rays: bursts gamma rays: theory

© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998
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