In August this year, gravitational waves and electromagnetic radiation were first observed as a result of the fusion of two neutron stars. The documentation of the waves was made possible thanks to the "Lego" detector. Now, an international team of researchers, including Israeli researchers from the Hebrew University and Tel Aviv University, is revealing new radio observations that clarify what actually happened during the collision.

The gravitational wave event, named GW170817, which was discovered last August (and published during the month of October) is the first time that gravitational waves have been observed from neutron star fusion and also the first time electromagnetic radiation has been observed (almost across the spectrum including gamma Visible light and radio radiation) accompanying an event of gravitational waves.

This event provided a rare opportunity to explore the fusion of neutron stars. For example, observations in the field of visible light and infrared provided for the first time evidence that there is indeed the creation of heavy elements in the mergers of neutron stars (a process hitherto only predicted in theory). However, this event also provided new mysteries that have not yet been explored so far.

The prevailing theory is that in such an event of neutron star fusion, a lot of energy will be emitted in a focused and narrow beam (a narrow jet moving at a very high speed close to the speed of light). This jet is expected to penetrate the material around it, and when it is pointed in our direction we expect to see gamma radiation. For years scientists have speculated that this is the model that explains short gamma-ray bursts observed every few weeks.

Indeed, about two seconds after the discovery of gravitational waves, short gamma radiation was also observed. However, the gamma radiation observed was not typical. It was very weak relative to other short gamma-ray bursts observed so far (events without gravitational waves).

Scientists around the world have tried to explain the weak radiation by the fact that although a jet was created in the event, it is not directed directly at us but at a slight tilt. If this explanation is correct, then the theory tells us that we should see radio radiation now, several months after the initial discovery, should fade.

It was clear to an international team, including Dr. Assaf Harash, an experimental astrophysicist at the Hebrew University, that conducting radio observations over several months would help solve the mystery. Radio observations can be made during the day, so data on astronomical events can continue to be collected while not being viewed with other telescopes, such as optical telescopes. Over the past two months the location of the GW170817 event across the sky has become increasingly close to the sun, leading to the cessation of observations at all wavelengths except radio observations, while being critical for data collection.

Now, the team of researchers who carried out the radio observations claim (in an article published in Nature magazine) that in light of the new observations a jet was indeed formed, but it did not penetrate the material around it so the observed radiation did not come from the jet itself. The researchers found that radio radiation only became clearer for about 100 days after initial detection, rather than fading.

The radio observation indicates that the jet created at the event quickly lost its energy in favor of creating a shell around it, in accordance with a theoretical model developed by Prof. Zvi Piran of the Hebrew University, and Prof. Ehud Nakar and Uri Gottlieb of Tel Aviv University. This shell is a wide shell of material moving at high speed, but lower than the speed of the jet itself. In fact, the jet lost all its energy in favor of creating the shell around it, which led to the jet itself apparently ceasing to exist in a short time.

Radio observations thus shed new light on the model for the recently discovered gravitational wave event, as it is now clear that this event is not an event similar to a typical short gamma-ray burst, so the relationship previously proposed between neutron star fusion and short gamma-ray bursts is far from proven.

In contrast, it is now clear to us what radio radiation is expected from a merger of neutron stars, so in the future scientists around the world will know what to look for in order to find more such events.

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