Reverse Shocks in Short Gamma-Ray Bursts -- The case of GRB 160821B and prospects as gravitational-wave counterparts

The shock system that produces the afterglow to GRBs consists of a forward- and a reverse-shock. For short GRBs, observational evidence for a reverse-shock has been sparse, however, the afterglow to GRB 160821B requires a reverse-shock at early times to explain the radio observations. GRB 160821B is additionally accompanied by the best-sampled macronova without a gravitational-wave detection, and an interesting late-time X-ray afterglow behaviour indicative of a refreshed-shock. The presence of an observed reverse-shock in an on-axis short GRB means that the reverse-shock should be considered as a potential counterpart to gravitational-wave detected mergers. As a gravitational-wave counterpart, the afterglow to an off-axis GRB jet can reveal the jet structure -- a reverse-shock will exist in these structured jet systems and the signature of these reverse-shocks, if observed, can indicate the degree of magnetisation in the outflow. Here we show the case of GRB 160821B, and how a reverse-shock will appear for an off-axis observer to a structured jet.