Global Gravitational Wave Network Detects Another Neutron Star Collision

An international network of scientists has detected what appears to be gravitational ripples from a collision of two neutron stars - only the second time this has ever been recorded.

The new study, which involves researchers from the University of Strathclyde, confirms that the event on 25 April 2019, was likely the result of a merger of two neutron stars.

It was witnessed by only one gravitational-wave observatory network, the Laser Interferometer Gravitational-Wave Observatory (LIGO) in Livingston in the USA.

The combined mass of the two merging neutron stars appears to be three and a half times that of our Sun, making it the largest coalescing system that has ever been observed in our galaxy and one which challenges researchers' expectations.

The global gravitational-wave observatory network includes the National Science Foundation's LIGO in the USA and the European Virgo detectors, and has substantial support from UK research teams from the Institute of Gravitational Wave Astronomy, the University of Birmingham, Cardiff University’s Gravitational Physics Group, the University of Strathclyde and the University of Glasgow’s Institute for Gravitational Research.

Prof Stuart Reid from Strathclyde said: “It is fantastic to witness the second observation of neutron stars colliding, as we continue to listen to our Universe through gravitational waves. The fact that the combined mass of the two neutron stars is higher than we expected shows that there is much still to learn about the life and death of stars, and the Universe we live in.

The first detection of gravitational waves was a milestone in physics and astronomy. As well as confirming a major prediction of Albert Einstein’s 1915 theory of general relativity, it marked the beginning of the new field of gravitational-wave astronomy. 

Professor Sheila Rowan, Director of the University of Glasgow’s Institute for Gravitational Research, said: “We’re thrilled to have observed a second neutron star collision, less than two years after the first detection of this type of event, and we’ve been pleasantly surprised by the unexpectedly high mass of the binary.

“It’s a reminder that there’s still so much that gravitational wave astronomy has yet to reveal about how our universe works.

Neutron star pairs are thought to form either early in life, when companion massive stars successively die one by one—or they are thought to come together later in life within dense, busy environments. The LIGO data for the April 25 event do not indicate which of these scenarios is more likely, but suggest that more data and new models are needed to explain the unexpectedly high mass.

LIGO became the first observatory to directly detect gravitational waves in 2015 and in that instance, the waves were generated by the fierce collision of two black holes. Since then, LIGO and Virgo have registered dozens of additional candidate black hole mergers.

Since the detectors first started operation in September 2015, they have completed two observation runs. During these runs they have detected gravitational waves from a total of ten stellar-mass binary black hole mergers – compact objects likely formed by the gravitational collapse of massive stars. They have also detected one binary neutron star coalescence – generated by two neutron stars spiraling into each other.

Around 1,300 scientists from around the world participate in the effort through the LIGO Scientific Collaboration, with the UK’s contribution to the collaborations funded by the Science and Technology Facilities Council (STFC).

An animation of the merger event is available.

The study, submitted to the Astrophysical Journal Letters, is authored by an international team from the LIGO Scientific Collaboration and the Virgo Collaboration, the latter of which is associated with the Virgo gravitational-wave detector in Italy.

January 2020

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