Astrophysicists at the University of Glasgow are celebrating the detection of gravitational wave signals likely to be caused by the crashing of two neutron stars and what could be the first evidence of the collision of a neutron star and a black hole.

The University of Glasgow researchers are key partners in the international scientific collaboration which  made the new detections – the National Science Foundation's Laser Interferometer Gravitational-Wave Observatory (LIGO), based in the United States, and Virgo, based in Italy.

The LIGO-Virgo detector network began its latest observing run on April 1, the first run since upgrade work improved their sensitivity to the tiny ripples in spacetime known as gravitational waves. Those upgrades allow the detectors to pick up signals from an even larger volume of the universe than before.

On April 25, 2019, the detectors registered gravitational waves from what appears likely to be a collision between two neutron stars—the dense remnants of massive exploded stars – which occurred about 500 million light-years from Earth.

One day later, on April 26, the LIGO-Virgo network spotted another candidate source from an event about 1.2 billion light-years away. While the signal was weak and thus requires further investigation, initial analysis of the signal suggests one possible cause could be an event which has never been witnessed before: the collision of a neutron star and black hole.

The University of Glasgow’s contribution to the LIGO detector is supported by the UK’s Science and Technology Funding Council (STFC). Alongside partner institutions including the Universities of Birmingham and Cardiff, Glasgow researchers play a key role in the sophisticated data analysis which underpins each detection. They also led the development of the silica mirror suspensions which are a critically important part of the detector technology.

Professor Sheila Rowan, director of the University of Glasgow’s Institute for Gravitational Research, said: “These two new triggers, which came just weeks after the upgraded detectors started their search for gravitational waves again, are further evidence that our universe regularly rings with the aftershocks of colossal astronomical events.

“We’d been deaf to those sounds before LIGO and Virgo equipped us with the opportunity to hear them, and each event gives invaluable new data points to expand our understanding of our cosmos. We’re very excited to see what else the detectors will find over the remaining months of this observing run.”

While neutron star collisions cause gravitational waves, their impacts also release light across the electromagnetic spectrum. In 2017, LIGO-Virgo’s first-ever detection of a gravitational wave collision was also observed by many conventional telescopes. This time, telescopes around the world once again raced to track the sources and pick up the light expected to arise from these mergers. Hundreds of astronomers eagerly pointed telescopes at patches of sky suspected to house the signal sources. However, at this time, neither of the sources has been pinpointed.

In addition to the two new candidates involving neutron stars, the LIGO-Virgo network has, in this latest run, spotted three possible black hole mergers. In total, since making history with the first-ever direct detection of gravitational waves in 2015, the network has spotted evidence for two neutron star mergers, 13 black hole mergers, and one possible black hole-neutron star merger.

Patrick Brady, spokesperson for the LIGO Scientific Collaboration and a professor of physics at the University of Wisconsin-Milwaukee, said: "The universe is keeping us on our toes. We're especially curious about the April 26 candidate.

“Unfortunately, the signal is rather weak. It's like listening to somebody whisper a word in a busy café; it can be difficult to make out the word or even to be sure that the person whispered at all. It will take some time to reach a conclusion about this candidate."

National Science Foundation director France Cordova said: "NSF's LIGO, in collaboration with Virgo, has opened up the universe to future generations of scientists.

"Once again, we have witnessed the remarkable phenomenon of a neutron star merger, followed up closely by another possible merger of collapsed stars. With these new discoveries, we see the LIGO-Virgo collaborations realizing their potential of regularly producing discoveries that were once impossible. The data from these discoveries, and others sure to follow, will help the scientific community revolutionize our understanding of the invisible universe."

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First published: 2 May 2019