GW170817 was the gravitational wave detection of a binary neutron-star merger in the galaxy NGC 4993. It was thought that the detected gamma-ray, X-ray and radio emission was due to an ultra-relativistic jet launched during the merger, directed away from our line of sight. However, monitoring by a network of radio telescopes, including the Australia Telescope Compact Array, find that the radio light curve of GW170817 has no direct signature of an off-axis jet afterglow. The radio data can be modelled by a mildly relativistic wide-angle outflow moving towards us, possibly a cocoon of material that breaks out when a jet transfers its energy to the dynamical ejecta. The image above is taken from a simulation of a neutron-star merger creating such a broad outflow, or cocoon. The data and modelling were presented in a paper in Nature at the end of 2017: see the press release for more details.

Image credit: Ehud Nakar (Tel Aviv), Ore Gottlieb (Tal Aviv), L. Singer (NASA), Mansi Kasliwal (Caltech) and the GROWTH collaboration.