Merger delay time distribution of extended emission short GRBs

Abstract

The most popular progenitor model for short-duration gamma-ray bursts (sGRBs) is the merger of two compact objects. The spatial and temporal coincidence of GRB 170817A with the binary neutron star merger GW170817 has strengthened this model. However, the short GRB population exhibits a certain diversity: some bursts display an extended emission (EE), continuing in soft gamma-rays for a few hundreds of seconds post the short initial pulse. It is currently unclear whether the origin of such bursts is in compact object mergers. Within the merger model, the redshift (z) distribution of short GRBs is influenced by the merger delay time, i.e. time elapsed between the merger and the formation of the binary star system, which is dominated by the time-scale for gravitational wave losses during the compact binary phase. We examine redshift distributions of short GRBs with EE to see whether their formation channel requires considerable delay post the star formation episode. Our results show that the z-distribution of EE bursts is consistent with the merger model. We attempted to compare the delay time distribution of the EE and the non-EE short bursts. However, no statistically significant difference could be seen within the limited sample size. For general short GRB z-distribution, we find that luminosity functions extending to lower values prefer shorter delay times compared to those terminated at higher luminosities.