New insights into the radio emission from the merger of two neutron stars, which also produced gravitational waves, are reported online in Nature this week.
GW170817, the first gravitational-wave detection from a merger of two neutron stars, was accompanied by radiation across the electromagnetic spectrum. This radiation was localized to a galaxy called NGC 4993, located 40 million parsecs (130 million light years) away. The radio and X-ray afterglows associated with GW170817 had a delayed onset, with a peak about 150 days after the merger, followed by a relatively rapid decline. So far, various models have been proposed to explain the afterglow emissions, including a choked jet - where a jetted outflow is unable to cleanly escape the neutron-rich material ejected during the merger - and a successful jet surrounded by a wide angle outflow (called a 'cocoon', in which energy from a jet is deposited into the expanding material ejected from the merger). However, it has not been possible to determine which model is correct from the observational data collected so far.
Using high-angular-resolution radio observations, Kunal Mooley, Adam Deller, Ore Gottlieb, and colleagues show that the source of radio emissions associated with GW170817 exhibited superluminal apparent motion (which implies that its actual motion was close to the speed of light) between 75 and 230 days after the merger. The authors suggest that the early radio emission was powered by a wide-angle outflow (the ‘cocoon’), but that the later emission was most probably dominated by an energetic, narrow jet. These observations support the successful jet model to explain the afterglow emission from the binary neutron-star merger.