Study of Gamma‐Ray Burst Binary Progenitors

Abstract

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.

Gravitational Radiation from Gamma‐Ray Burst Progenitors

“…The estimates on the formation rates by Belczynski et al (2002a) are consistent with the results of Fryer et al (1999a) and within the uncertainty range in Table 1 in most of their models. Although some of their models predict higher formation rates by a factor of a few than the upper limits in Table 1, the uncertainty range of the distances are similar because the distances are rather insensitive to the rate d / R À1=3 .…”

“…Recently, Fryer et al (1999a) and Belczynski, Bulik, & Rudak (2002a) have estimated the formation rate of these progenitors by using population synthesis methods. The results of Fryer et al (1999a) are summarized in Table 1, where the standard values of the formation rates and the uncertainty ranges are listed.…”

“…The more massive star (primary) evolves off the main sequence and forms a compact object (most likely a black hole) in a supernova explosion (Fryer et al 1999a;Belczynski et al 2002a). When the secondary, in turn, evolves off the main sequence and expands, the compact object enters its companion's hydrogen envelope.…”

Gravitational lensing as a probe of compact object populations in the Galaxy

“…The single star evolution is modeled using the modified formulae of Hurley et al (2000) as presented by Belczynski et al (2005) and we use the StarTrack population synthesis code (Belczynski et al 2002a) to model binary evolution. The StarTrack population synthesis code was initially developed for the study of double compact object mergers in the context of gamma-ray bursts progenitors (Belczynski et al 2002a) and gravitational-wave inspiral sources (Belczynski et al 2002b).…”

“…The StarTrack population synthesis code was initially developed for the study of double compact object mergers in the context of gamma-ray bursts progenitors (Belczynski et al 2002a) and gravitational-wave inspiral sources (Belczynski et al 2002b). In recent years StarTrack has undergone major updates and revisions in the physical treatment of various binary evolution phases.…”