Viewing Short Gamma-Ray Bursts From a Different Angle

Abstract

The detection of a faint, short gamma-ray burst (GRB 170817A) in coincidence with the gravitational wave (GW 170817) detection by LIGO/Virgo is at odds with the expected known luminosity and redshift distribution of short GRBs (sGRB). Examining the observer-frame parameter space of all Fermi-GBM sGRBs shows that the sGRB associated with GW 170817 is extreme in its combination of flux, spectral softness and temporal structure. We identify a group of similar GRBs, one of which has been associated to a bright galaxy at 75 Mpc. We speculate that a good fraction of the previously detected faint sGRBs is not at large redshifts, but local, at redshift smaller than 0.1, seen off-axis. We incorporate off-axis emission in the estimate of the rates of sGRBs, and predict that a large fraction of future GW-detections of NS-NS mergers will be accompanied by faint γ-ray emission, contrary to previous thinking. The much wider gamma-ray emission cone from NS-NS mergers also implies a higher deadly rate of γ-rays for extraterrestrial life in the Universe.