First confirmed neutron star merger re-appears from behind the Sun

A research team including astronomers and astrophysicists at the Universities of Warwick and Leicester had to wait over 100 days for the sight of the first confirmed neutron star merger to re-appear from behind the glare of the Sun.

They were rewarded with the first confirmed visual sighting of a jet of material that was still streaming out from the merged star exactly 110 days after that initial cataclysmic event was observed. Their observations confirm a key prediction about the aftermath of neutron star mergers.

The binary neutron star merger GW170817 occurred 130 million light years away in a galaxy named NGC 4993. It was detected in August 2017 by the Advanced Laser Interferometer Gravitational-Wave Observatory (Adv-LIGO), and by Gamma Ray Burst (GRB) observations, and then became the first ever neutron star merger to be observed and confirmed by visual astronomy.

After a few weeks the merged star passed behind the glare of our sun leaving it effectively hidden from view for astronomers until it re-emerged from that glare 100 days after the merger event. It was at this point that the research team were able to use the Hubble Space Telescope to see that the star was still generating a powerful beam of light in a direction that, while off centre to the Earth, was starting to spread out in our direction.

These observations confirm the prediction made by the second lead author of the paper, Dr Gavin Lamb from our Department of Physics and Astronomy, that these types of events will reveal the structure of these jets of material travelling close to the speed of light: "The behaviour of the light from these jets, how it brightens and fades, can be used to determine the velocity of the material throughout the jet. As the afterglow brightens we are seeing deeper into the jet structure and probing the fastest components. This will help us understand how these jets of material, travelling close to the speed of light, are formed and how they are accelerated to these phenomenal velocities. If this jet had been pointing towards us, we would have seen a very bright burst of gamma-rays."

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