The Afterglows to Short-duration Gamma-ray Bursts in the Gravitational Wave Era

Gravitational wave (GW) detected neutron star mergers provide a unique opportunity to observe their electromagnetic counterparts. Such mergers are thought to be the progenitor system for the short-duration, less than 2 s, class of gamma-ray bursts (GRBs). The highly collimated outflows that produce the cosmological population of GRBs are uniquely observed on-axis, or within the outflows opening angle. However, for a GW detected system the typical inclination is ~38 degrees and the GRB producing outflow will be viewed off-axis. Although GRBs are not expected to be observable at such high inclinations, for the relatively nearby GW detectable neutron star mergers, the afterglow that accompanies a typical short GRB will be seen at late-times as the outflow decelerates and the emission is beamed over a wider angle. The afterglow viewed off-axis can be used to determine the angular energy and/or velocity structure of the blastwave produced as the outflow decelerates, providing clues to the dynamics of the outflow and the environment through which it will have propagated. Additionally, early radio observations may detect reverse shock emission that can probe the magnetic field towards the merger remnant.