On last August, gravitational waves together with electromagnetic radiation originating from a merger of two neutron stars, have been observed for the first time. The discovery of the gravitational waves has been made possible thanks to the advanced LIGO experiment. Now, a team of international scientists, including researchers from The Hebrew University and Tel-Aviv University, reveals new radio observations that shed new light on what has occurred during the neutron star merger.
This event, named GW170817, and that exhibited electromagnetic radiation across the spectrum (including gamma-rays, X-rays, ultra-violet light, and optical and infra-red light) provided a unique opportunity to study neutron star merger like never before. For example, optical and infra-red observations of the event provided evidence (for the first time) that heavy elements indeed form in this type of mergers.
The common theory suggests that in this type of an event, a large amount of energy will be carried out by a relativistic narrow jet (moving close to the speed of light). This jet is expected to penetrate the material around it (that is ejected during the merger process) and when the jet is pointing towards us, we expect to see gamma-ray emission. For many years, this was the model that has been used to explain short gamma-ray bursts that are observed once every few weeks (without any associated gravitational events).
Indeed, about two seconds after the discovery of GW170817, a short gamma ray burst emission has been observed. However, the emission was atypical compared to previous short bursts, as it was much fainter.
Scientists around the world tried to explain the weak gamma-ray emission by suggesting that the narrow relativistic jet is not pointing straight at us, but it is slightly off-axis. If this is true, then this model predicts that the radio emission from such a jet should decay by now, a few months after the merger.
It was therefore clear to the international radio observing team (that includes Dr. Assaf Horesh, an experimental astrophysicist from the Hebrew University) that prolonged radio observations can help solve this mystery. Also, radio observations can be undertaken during the day. This was important since GW170817 position on the sky has gotten closer to the sun these last few months, causing the observations in all other wavelengths (besides the radio) to cease, during a critical phase in gathering information on the event.
Now the team of researchers who performed the radio observations claim (in their new paper published in Nature) that in light of the new radio measurements a narrow jet was formed, but we have not seen it, as the radio emission has continued to rise over the last 100 days instead of fading away.
The jet quickly lost its energy as it deposited it into a wide-angle cocoon, created by the jet itself (according to a theoretical model developed by Prof. Tsvi Piran from the Hebrew University and Prof. Ehud Nakar and Ore Gottelib from Tel-Aviv University). This cocoon moves at a fast velocity but as fast as the jet that formed it. In fact, the jet lost all of its energy and quickly ceased to exist.
The radio observations, therefore, shed new light on the processes that take place when two neutron stars merge. Moreover, due to this result, it is clear that the connection between neutron star mergers and normal short gamma-ray bursts is far from being proven. However, at least now we know what to expect from neutron star mergers and what to look for in future events