Looking for GRB progenitors

Using stellar binary population synthesis code we calculate the production rates and lifetimes of several types of possible GRB progenitors. We consider mergers of double neutron stars, black hole neutron stars, black hole white dwarfs and helium star mergers. We calibrate the results with the measured star formation rate history. We discuss the viability of each GRB model, and alternatively assuming that all bursts are connected with one model we constrain the required collimation of GRBs. We also show the importance of widely used evolutionary parameters on the merger rates of calculated binary populations.Comment: 5 pages, 2 figures, Latex with aipproc.sty, Proc. of the 5th Huntsville Gamma Ray Burst Symposium, Oct. 1999, ed. R.M. Kippen, AI

First Stellar Binary Black Holes: Strongest Gravitational Wave Burst Sources

Evolution of first population of massive metal-free binary stars is followed. Due to the low metallicity, the stars are allowed to form with large initial masses and to evolve without significant mass loss. Evolution at zero metallicity, therefore, may lead to the formation of massive remnants. In particular, black holes of intermediate-mass (100-500 Msun) are expected to have formed in early Universe, in contrast to the much lower mass stellar black holes (10 Msun) being formed at present. Following a natural assumption, that some of these Population III stars have formed in binaries, the physical properties of first stellar binary black holes are presented. We find that a significant fraction of such binary black holes coalesces within the Hubble time. We point out that burst of gravitational waves from the final coalescences and the following ringdown of these binary black hole mergers can be observed in the interferometric detectors. We estimate that advanced LIGO detection rate of such mergers is at least several events per year with high signal to noise ratio (>10).Comment: 4 pages, 3 figures, submitted to ApJ Letter

Study of Gamma Ray Burst Binary Progenitors

Recently much work in studying Gamma-Ray Burst has been devoted to revealing the nature of outburst mechanism and studies of GRB afterglows. These issues have also been closely followed by the quest for identifying GRB progenitors. In this paper we consider the proposed binary star progenitors of GRBs: white dwarf neutron star binaries, white dwarf black hole binaries, helium core neutron star mergers, helium core black hole mergers, double neutron stars and neutron star black hole binaries. Using population synthesis methods we calculate merger rates of these binary progenitors and we compare them to the observed BATSE GRB rate. 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 NS-NS and NS-BH mergers with respect to their hosts are significantly different. This may allow to distinguish between these two progenitor models, once current and near future missions, such as HETE-II or SWIFT, measure the locations of short GRBs.Comment: 26 pages, 12 figures, submitted to Ap