Constraining Short Gamma-Ray Burst Jet Properties Using Coincident Gravitational Wave and Electromagnetic Detections

The simultaneous detection of GW170817 and GRB170817A ushered in the era of multi messenger astronomy, confirming the hypothesis that binary neutron star mergers are the progenitors of at least some short gamma-ray bursts. GRB prompt emission is thought to be highly beamed, but determining the opening angle of the jet traditionally relies on the observation of a jet break in the afterglow since the inclination angle of the system is unknown. Coincident gravitational wave observations of short GRBs from binary neutron star mergers, however, provide an independent measurement of the inclination angle and distance to the source. We describe a Bayesian method for determining the morphology of short GRBs using coincident electromagnetic and gravitational-wave observations. In particular, we show that is possible to measure the gamma-ray burst opening angle, Lorentz factor, and total energy for a particular event, and that by studying a population of such coincident detections the distributions of these parameters can also be inferred.