Short Gamma-ray Bursts and their afterglows in the Gravitational Wave era

The leading progenitor candidate for short Gamma-ray Bursts (GRBs) is the merger of binary neutron stars or neutron star black holes and the cosmological short GRB population is therefore thought to trace the population of evolved compact binary systems. Material that is ejected during the violent merger of a neutron star binary results in a thermal transient powered by the radioactive decay of heavy elements created via r-process nucleosynthesis. These transients, known variously as macronova or kilonova, should accompany most neutron star mergers. The identification macro/kilonova in the afterglows to cosmological short GRBs provides the 'smoking gun' evidence for the link between short GRBs and compact binaries. The association of a short GRB and a macro/kilonova with the gravitational wave detected binary neutron star merger, GW170817, has confirmed that these systems can be responsible for some fraction of the observed short GRB population, however, GRB 170817A and its afterglow was unlike any other. The highly collimated outflows (jets) that produce the cosmological population of GRBs are uniquely observed on-axis, or within the outflows opening angle (typically <10 degrees from the system rotational axis), however, for a GW detected system the typical inclination is ~30 degrees and the GRB producing outflow will be viewed off-axis. GW detected mergers therefore provide a unique opportunity to perform targeted searches for electromagnetic transients from off-axis GRBs where for an off-axis observer, the temporal behaviour of the afterglow lightcurve can be used to probe the energy and velocity structure of the jet and late-time radio imaging used as confirmation. However, the structured jet scenario used to explain the late time afterglow to GW170817 is not a unique solution.

I will discuss the breakthrough observations of the afterglow following GW170817 and ask the question, 'how common are GRB 170817A-like events?'

Additionally, I will present an alternative to the structured jet model for the afterglow to GRB 170817A and show how this 'new' model has been used to explain variability in the late-time afterglows to cosmological GRBs and reveal the macro/kilonova in the afterglow to GRB 160821B.

Event resources