Artist’s impression of jets of material from first confirmed neutron star merger.
Artist’s impression of jets of material from first confirmed neutron star merger. Image: Mark Garlick/University of Warwick

The research team was rewarded with the first confirmed visual sighting of a jet of material that was still streaming out from merged star exactly 110 days after that initial cataclysmic merger event was first 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 location of the merger remnant then passed behind the glare of our sun leaving it effectively hidden from astronomers until it re-emerged from that glare 100 days after the merger event. It was at that point that the research team were able to use the Hubble Space Telescope to see 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.

Their research has just been published in a paper entitled: “The optical afterglow of the short gamma-ray burst associated with GW170817” in Nature Astronomy's website

Beginning of multi-messenger astrophysics

Stephan Rosswog, Professor at The Oskar Klein Centre and Department of Astronomy at Stockholm University, is one of the authors of the article. He says that 2017 marks the beginning of multi-messenger astrophysics: for the first time ever the gravitational waves from the merger of two ultra-dense stars, so-called neutron stars, were detected. Directly after the burst, a flash of gamma-rays was observed from the same location and in the following weeks a firework all across the electromagnetic spectrum was observed.

Major leaps for physics

This combined "multi-messenger observation" has lead to major leaps forward for various fields in physics. It showed that neutron star mergers produce gravitational waves as predicted by Einstein's Theory of Relativity, that gravitational waves travel at the speed of light and it allowed for a new measurement of the expansion rate of the Universe. These observations further showed that neutron star mergers produce the heaviest elements in the Universe, such as gold and platinum. For all of these reasons, this event was celebrated by the Science Magazine as "The Breakthrough of the year 2017".

Amazing times for astrophysics

There was, however, a scientific controversy, whether the observed flash of gamma-rays was really due to a "classical" gamma-ray burst. Such a burst produces its radiation in an ultra-relativistic, highly collimated outflow. The alternative suggestion is that the radiation comes from a more slowly moving cloud of material called a "cocoon", but the event would not have looked like a bright gamma-ray burst to any observer in the Universe. The new observations that are published in a Nature Astronomy paper now find good indications that the gamma-radiation was indeed produced by the ultra-relativistic outflow and if we had seen this event from a different viewing angle, it would have looked like classical, very bright gamma-ray burst.

"These are amazing times for astrophysics: many puzzles that have been around for decades are now being conclusively solved. Last August, our theoretical ideas how heavy elements are produced in the cosmos, were confirmed by the first multi-messenger observation of a neutron star merger. Now the same event teaches us about the inner workings of gamma-ray bursts. We are all very excited and eagerly looking forward to seeing more such events." says professor Stephan Rosswog.

Article in Nature Astronomy: The optical afterglow of the short gamma-ray burst associated with GW170817