A cocoon after the kilonova

Scenarios after the kilonova (Image NRAO/AUI/NSF: D. Berry)
Scenarios after the kilonova (Image NRAO/AUI/NSF: D. Berry)

An article published in the journal “Nature” describes a research on the consequences of the merger between two neutron stars observed in the emission of both electromagnetic and gravitational waves. A team of researchers led by Kunal Mooley of the US National Radio Astronomy Observatory (NRAO) used the Very Large Array (VLA) together with the Australia Telescope Compact Array and the Giant Metrewave Radio Telescope in India for three months from the beginning of September to detect the radio waves emitted by the event at the origin of the gravitational waves recorded on August 17, 2017 in the event labeled as GW170817.

That event wasn’t the first detection of gravitational waves but represented a leap forward in a new branch of astronomy because it was the first observed also in electromagnetic waves with a considerable amount of instruments involved and very different electromagnetic frequencies. The combined work between the LIGO/VIRGO collaboration and a number of scientific organizations was an extraordinary event in the history of research. However, that event left several issues open for further research.

The GW170817 event caused an explosion called kilonova, but what consequences did it have? The authors of this new research tried to understand it by observing with three radio telescopes the area where the merger between two neutron stars occurred. In recent years, various theoretical models were proposed but were based on a concept that until that event was also theoretical. Finally a kilonova was identified and subsequent observations have provided some answers.

First of all, the radio waves generated by the GW170817 event were detected together with X-rays only on September 2, over two weeks after the gravitational waves and this is already a significant fact. Another key factor in this research is the intensification of these electromagnetic emissions with the passing of days.

According to the researchers, these characteristics allowed to understand what happened after the kilonova. The data collected from the many observations made allowed to establish that the jets generated by the disk formed by the materials ejected after the merger between the two neutron stars were not aligned with the Earth. This explains why the radio waves and X-rays emissions were detected well after the gravitational waves.

The doubts were between two different scenarios. In the scenario shown in the left part of the image, a jet of material that moves at almost the speed of light is projected from the area of collision against a sphere of material initially ejected by the explosion. If observed from an angle off-axis with respect to the jet, the long-term emission of radio waves and X-rays should get weaker.

In the scenario shown in the right part of the image, the jet can’t burst through the shell of explosion debris, instead it sweeps up material into a large “cocoon”, which absorbs the energy of the jet and emits X-rays and radio waves over a wider angle. In this case, the emissions of radio waves and X-rays grow in intensity.

The detections, including the most recent X-ray measurements made with NASA’s Chandra space observatory, confirm the cocoon scenario. At the heart of that cocoon a black hole probably formed and the materials ejected in the kilonova started getting attracted back to it.

The journal “Science” crowned the GW170817 event as the scientific breakthrough of the year for all the progress it brought and will bring to astronomy and astrophysics. The research that confirmed the cocoon model is one of the first advances and among the implications there’s the possibility that in the future it will be possible to detect many more mergers of that type to develop old and new astronomy.

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