Gavin Paul Lamb

Neutron stars are the dense remnants of massive stars that have ended their lives in a supernova. Neutron stars are regularly found to be in binary systems and where these stars orbit close to one another, their mass is so concentrated that the orbital motion of the binary loses energy and angular momentum via gravitational wave radiation - 'ripples', or disturbances in the curvature of space-time, travel away from the source at the speed-of-light. Over time the stars will get closer together, and their orbital frequency will increase, eventually the tidal forces between the neutron stars is so strong that they tear each other apart and merge. The merger of a neutron star binary system produces detectable gravitational wave emission.

These mergers result in explosions that are so extreme that very heavy elements are fused from the material. The resultant heavy elements radioactively decay producing a thermal flash of light known as a kilo/macronova.
In addition to the kilo/macronova, the hyper-accretion of material via the merger remnant launches a highly collimated jet of material aligned with the rotational axis of the system. These narrow bipolar jets have a velocity close to the speed of light.

Energy dissipated with these jets results in a brief flash of gamma-ray photons known as a gamma-ray burst (GRB). These jets will expand into the surrounding space and as they do they sweep-up matter that results in a shock system that produces an afterglow that shines over a broad frequency range from radio, through optical to X-ray frequencies. In August 2017, the simultaneous observation of gravitational waves and a GRB marked the first joint electromagnetic and gravitational wave observed transient. This event was later accompanied by a thermal kilo/macronova and at after ten days by a broadband afterglow. The afterglow to this event is the first confirmed case of a GRB jet seen 'off-axis', or at higher inclinations than the classically 'on-axis' GRB population and gave a unique opportunity to map the structure of the jet via the afterglow lightcurve.

The late-time electromagnetic afterglow that followed the near simultaneous gravitational wave detected merger and GRB, GW170817 and GRB 170817A respectively, were initially explained as revealing the lateral energy structure of these collimated jets, however, a new interpretation of the data shows that the observed lightcurves are consistent with a narrow and uniform jet that is refreshed at late times by a period of energy injection. Such energy injection has been observed in the afterglows to some 'on-axis' GRBs, systems that are analogous to GW170817 but viewed with the jet pointing towards the observer. The afterglow shock system can be energised (or refreshed) by long-lived central engine activity, which produces variability in the afterglow.

One example of a refreshed/energised afterglow is that of GRB 160821B, where the inclusion of energy injection in the modelling of the afterglow is required to explain the broadband temporal and spectral features. For the afterglow to GRB 160821B, inclusion of energy injection for the afterglow clearly reveals the associated kilo/macronova. Here we show how such a refreshed shock afterglow can explain the observed temporal behaviour of the late-time electromagnetic emission to GW170817/GRB 170817A. Via refreshed shock model fitting to the data from the late afterglow to GRB 170817A, the energetics of the jet are found to be consistent with the 'on-axis' short gamma-ray burst population, whereas the initially favoured, structured jet models result in jet energies that are an outlier in the short-duration gamma-ray burst population.