Study of gamma-ray burst binary progenitors

Recently much work in studying gamma-ray bursts (GRBs) has been devoted to revealing the nature of outburst mechanisms and to studies of GRB afterglows. These issues have also been closely followed by the quest to identify GRB progenitors. Several types of progenitors have been proposed for GRBs: the most promising objects seem to be collapsars, compact object binaries, mergers of compact objects with helium cores of evolved stars in common envelope episodes, and also the recently discussed connection of GRBs with supernovae. In this paper we consider the binary star progenitors of GRBs: white dwarf neutron star (WD-NS) binaries, white dwarf black hole (WD-BH) binaries, helium core neutron star (He-NS) mergers, helium core black hole (He-BH) mergers, and double neutron star (NS-NS) and neutron star black hole binaries (NS-BH). Using population synthesis methods we calculate merger rates of these binary progenitors and compare them to the observed BATSE GRB rate. For the binaries considered, we also calculate the distribution of merger sites around host galaxies and compare them to the observed locations of GRB afterglows with respect to their hosts. We find that the rates of binary GRB progenitors in our standard model are lower than the observed GRB rates if GRBs are highly collimated. However, the uncertainty in the population synthesis results is too large to make this a firm conclusion. Although some observational signatures seem to point to collapsars as progenitors of long GRBs, we find that mergers of WD-NS, He-NS, He-BH, and NS-NS systems also trace the star formation regions of their host galaxies, as it is observed for long GRBs. We also speculate about possible progenitors of short-duration GRBs. For these, the most likely candidates are still mergers of compact objects. We find that the locations of NS-NS and NS-BH mergers with respect to their hosts are significantly different. This may allow us to distinguish between these two progenitor models once current and near future missions, such as HETE-2 or Swift, measure the locations of short GRBs.