Formation Rates of Black Hole Accretion Disk Gamma‐Ray Bursts

While many models have been proposed for GRBs, those currently favored areall based upon the formation of and/or rapid accretion into stellar mass blackholes. We present population synthesis calculations of these models using aMonte Carlo approach in which the many uncertain parameters intrinsic to suchcalculations are varied. We estimate the event rate for each class of model aswell as the propagation distance for those having significant delay betweenformation and burst production, i.e., double neutron star (DNS) mergers andblack hole-neutron star (BH/NS) mergers. For reasonable assumptions regardingthe many uncertainties in population synthesis, we calculate a daily event ratein the universe for i) merging neutron stars: ~100/day; ii) neutron-star blackhole mergers: ~450/day; iii) collapsars: ~10,000/day; iv) helium star blackhole mergers: ~1000/day; and v) white dwarf black hole mergers: ~20/day. Therange of uncertainty in these numbers however, is very large, typically two tothree orders of magnitude. These rates must additionally be multiplied by anyrelevant beaming factor and sampling fraction (if the entire universal set ofmodels is not being observed). Depending upon the mass of the host galaxy, halfof the DNS and BH/NS mergers will happen within 60kpc (for a Milky-Way massedgalaxy) to 5Mpc (for a galaxy with negligible mass) from the galactic center.Because of the delay time, neutron star and black hole mergers will happen at aredshift 0.5 to 0.8 times that of the other classes of models. Information isstill lacking regarding the hosts of short hard bursts, but we suggest thatthey are due to DNS and BH/NS mergers and thus will ultimately be determined tolie outside of galaxies and at a closer mean distance than long complex bursts(which we attribute to collapsars).Comment: 57 pages total, 23 figures, submitted by Ap