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Crashing neutron stars unlock secrets of the Universe – thanks to UK tech

In a galaxy far away, two dead stars begin a final spiral into a massive collision. The resulting explosion unleashes a huge burst of energy, sending ripples across the very fabric of space. In the nuclear cauldron of the collision, atoms are ripped apart to form entirely new elements and scattered outward across the Universe.

It could be a scenario from science fiction, but it really happened 130 million years ago -- in the NGC 4993 galaxy in the Hydra constellation, at a time here on Earth when dinosaurs still ruled and flowering plants were only just evolving.

Yesterday, dozens of UK scientists and their international collaborators representing 70 observatories worldwide announced the detection of this event and the significant “scientific firsts” it has revealed about our Universe.

Those ripples in space finally reached Earth at 1.41pm UK time, on Thursday 17 August 2017, and were recorded by the twin detectors of the US-based Laser Interferometer Gravitational-wave Observatory (LIGO) and its European counterpart Virgo.

A few seconds later, the gamma-ray burst from the collision was recorded by two specialist space telescopes, and over following weeks, other space- and ground-based telescopes recorded the aftermath of the massive explosion. UK developed engineering and technology is at the heart of many of the instruments used for the detection and analysis.

Dr John Veitch, who is co-chair of LIGO’s Compact Binary Coalescence Search Group and Research Fellow at the University of Glasgow’s School of Physics and Astronomy and played a leading role in the GW170817 data analysis said: “One key difference between the gravitational wave signals from binary black holes and binary neutron stars is that neutron stars are many times lighter than black holes. This means that the gravitational wave signal from neutron stars linger for a much greater period in the detector – for around 100 seconds as opposed to just a fraction of a second for binary black holes. A longer signal means we can glean much more information about the source.”

Studying the data confirmed scientists’ initial conclusion that the event was the collision of a pair of neutron stars – the remnants of once gigantic stars, but collapsed down into approximately the size of a city.

UK Science Minister, Jo Johnson, yesterday said “Today’s announcement of the latest detection of gravitational waves is another important development in our understanding of the universe which has been made possible by UK research and technology.

“The recent awarding of the Nobel Prize for Physics to gravitational waves research is clear recognition of the importance of this area. The UK plays a significant role in these detections, enabling us to continue building our reputation as a world leader in science and innovation which is a core part of our Industrial Strategy.”

There are a number of “firsts” associated with this event, including the first detection of both gravitational waves and electromagnetic radiation (EM) - while existing astronomical observatories “see” EM across different frequencies (eg, optical, infra-red, gamma ray etc), gravitational waves are not EM but instead ripples in the fabric of space requiring completely different detection techniques. An analogy is that LIGO and Virgo “hear” the Universe.

The announcement also confirmed the first direct evidence that short gamma ray bursts are linked to colliding neutron stars. The shape of the gravitational waveform also provided a direct measure of the distance to the source, and it was the first confirmation and observation of the previously theoretical cataclysmic aftermaths of this kind of merger - a kilonova.

Additional research papers on the aftermath of the event have also produced new understanding of how heavy elements such as gold and platinum are created by supernova and stellar collisions and then spread through the Universe. More such original science results are still under current analysis.

By combining gravitational-wave and electromagnetic signals together, researchers also used a new technique to measure the expansion rate of the Universe. This technique was first proposed in 1986 by University of Cardiff’s Professor Bernard Schutz.

UK astronomers using the VISTA telescope in Chile were among the first to locate the new source. "We were really excited when we first got notification that a neutron star merger had been detected by LIGO,” said Professor Nial Tanvir from the University of Leicester, who leads a paper in Astrophysical Journal Letters. “We immediately triggered observations on several telescopes in Chile to search for the explosion that we expected it to produce. In the end we stayed up all night analysing the images as they came in, and it was remarkable how well the observations matched the theoretical predictions that had been made."

Dr Kate Maguire, from Queen’s University Belfast was part of the team studying the burst of light from the smashing together of the two neutron stars “Using rapid-response triggering at some of the world’s best telescopes, we have discovered that this neutron-star merger scattered heavy chemical elements, such as gold and platinum, out into space at high speeds. These new results have significantly contributed to solving the long-debated mystery of the origin of elements heavier than iron in the periodic table."

Once the location of the collision was pin-pointed, scientists quickly maneuvered the Swift satellite to examine the aftermath with its X-ray and UV/optical telescopes.

“We didn’t detect any X-rays from the object, which was surprising given the gamma ray detection,” said Dr Phil Evans from the University of Leicester, lead-author of a paper published yesterday in Science. “But we did find bright ultra-violet emission, which most people were not expecting. This discovery helped us to pin down what happened after the neutron star collision was detected by LIGO and Virgo.”

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