2015MNRAS.448.3026W

We estimate the rate and the luminosity function of short (hard) Gamma-Ray Bursts (sGRBs) that are non-Collapsars, using the peak fluxes and redshifts of BATSE, Swift and Fermi GRBs. Following Bromberg et al., we select a sub-sample of Swift bursts which are most likely non-Collapsars. We find that these sGRBs are delayed relative to the global star formation rate (SFR) with a typical delay time of a 3-4 Gyr (depending on the SFR model). However, if two or three sGRB at high redshifts have been missed because of selection effects, a distribution of delay times of ∝1/t would be also compatible. The current event rate of these non-Collapsar sGRBs with Liso > 5x10⁴⁹erg/s is 4.1_-1.9^+2.3 Gpc^-3 yr^-1. The rate was significantly larger around z ∼ 1 and it declines since that time. The luminosity function we find is a broken power law with a break at 2.0_-0.4^+1.4 ×10^52  erg s^-1 and power-law indices 0.95_-0.1 2^+0.12 and 2.0_-0.8^+1.0. When considering the whole Swift sGRB sample we find that it is composed of two populations: one group (~60-80 percent of Swift sGRBs) with the above rate and time delay and a second group (~20-40 percent of Swift sGRBs) of potential `impostors' that follow the SFR with no delay. These two populations are in very good agreement with the division of sGRBs to non-Collapsars and Collapsars suggested recently by Bromberg et al. If non-Collapsar sGRBs arise from neutron star merger this rate suggest a detection rate of 3-100/yr by a future gravitational wave detectors (e.g. Advanced Ligo/Virgo with detection horizon on 300 Mpc), and a co-detection with Fermi (Swift) rate of 0.1-1/yr (0.02-0.14/yr). We estimate that about 4 ×10^5 (f_b^-1/30) mergers took place in the Milky Way. If 0.025M_☉ were ejected in each event this would have been sufficient to produce all the heavy r-process material in the Galaxy.