University of Glasgow researchers part of collaboration behind new gravitational wave detection

Researchers from the University of Glasgow are part of the international collaboration behind the detection of a gravitational wave signal which casts new light on the diversity of cosmic objects.

In a paper presented at a meeting of the American Physical Society on Friday 5 April, researchers from LIGO-VIRGO-Kagra collaboration revealed a remarkable new gravitational wave signal detected in May last year.

In May 2023, shortly after the start of the fourth LIGO-Virgo-KAGRA observing run, the LIGO Livingston detector observed a gravitational-wave signal from the collision of what is most likely a neutron star with a compact object that is 2.5 to 4.5 times the mass of our Sun.

Neutron stars and black holes are both compact objects, the dense remnants of massive stellar explosions. What makes this signal, called GW230529, intriguing is the mass of the heavier object. It falls within a proposed mass-gap between the heaviest known neutron stars and the lightest black holes.

The gravitational-wave signal alone cannot reveal the nature of this object. Future detections of similar events, especially those accompanied by bursts of electromagnetic radiation, could hold the key to solving this cosmic mystery.

Dr Christopher Berry, a senior lecturer in the Institute for Gravitational Research, said: "This discovery has an object in the mass gap between three and five solar masses. From observations of X-ray binaries, it had been hypothesised that black holes of this mass range don’t form because of how stars explode in supernovae.

"This observation illustrates that there are still mysteries around the death of stars and births of black holes still to be uncovered. Thanks to the increased sensitivity of our gravitational-wave detectors, we are now able to begin solving these mysteries."

Still from a numerical simulation consistent with GW230529. [Credit: I. Markin (Potsdam University), T. Dietrich (Potsdam University and Max Planck Institute for Gravitational Physics), H. Pfeiffer, A. Buonanno (Max Planck Institute for Gravitational Physics)] Researchers from the University of Glasgow’s School of Physics & Astronomy and Institute for Gravitational Research have been active within international gravitational-wave detection collaborations for decades. They developed the delicate mirror suspensions at the heart of the LIGO detectors, and recently trained the staff who rehung the mirrors as part of a series of upgrades to the LIGO detectors which made this detection possible.

Glasgow researchers are also part of the data analysis team which helps pick out the gravitational wave signals from the background noise of the Universe and decode the information they contain about the properties of the cosmic bodies which created the waves. A total of 44 researchers from the University of Glasgow are named among the authors of the new paper, reflecting the depth of the University’s involvement in the LIGO-Virgo-KAGRA Collaboration.

Before the detection of gravitational waves in 2015, the masses of stellar-mass black holes were primarily found using X-ray observations while the masses of neutron stars were found using radio observations. The resulting measurements fell into two distinct ranges with a gap between them from about 2 to 5 times the mass of our Sun. Over the years, a small number of measurements have encroached on the mass-gap, which remains highly debated among astrophysicists.

Analysis of the signal GW230529 shows that it came from the merger of two compact objects, one with a mass between 1.2 to 2.0 times that of our Sun and the other slightly more than twice as massive. While the gravitational-wave signal does not provide enough information to determine with certainty whether these compact objects are neutron stars or black holes, it seems likely that the lighter object is a neutron star and the heavier object a black hole. Scientists in the LIGO-Virgo-KAGRA Collaboration are confident that the heavier object is within the mass gap.

Gravitational-wave observations have now provided almost 200 measurements of compact-object masses. Of these, only one other merger may have involved a mass-gap compact object - the signal GW190814 came from the merger of a black hole with a compact object exceeding the mass of the heaviest known neutron stars and possibly within the mass gap.

Dr Daniel Williams, of the School of Physics & Astronomy, said: "While this first detection is exciting, it probably begs more questions than it answers. The signal we detected was not strong enough for us to determine the nature of the heavier object, so we don’t know if it’s an unexpectedly light black hole, or a surprisingly heavy neutron star.

"If we make more observations of events like this, we’ll hopefully eventually see one which is close enough that we can make more detailed measurements of this sort of collision. If the larger objects turn out to be neutron stars this would provide exciting new understanding of some of the most exotic objects we know about in the Universe. Similarly, as we continue to develop and improve the technologies in our detectors we will be able to make clearer measurements of signals like this."

Dr Rachel Gray from the Institute for Gravitational Research said: "In addition to the puzzles this event presents, it also hints at an exciting opportunity. We generally regard collisions between pairs of neutron stars as being the best source of multimessenger observations - where we observe gravitational waves alongside another type of radiation, like radio waves or light.

"However, neutron star-black hole collisions may also produce observable electromagnetic radiation, but only if the black hole is low mass. In cases like this one, where the two objects have fairly similar masses, the neutron star is disrupted by strong tidal forces, which provides an environment where light can be produced. One of the things we’re excited by from this event is that it may suggest that more gravitational-wave events could be accompanied by light than we’d previously anticipated."

The highly successful third observing run of the gravitational-wave detectors ended in spring 2020, bringing the number of known gravitational-wave detections to 90. Before the start of the fourth observing run O4 on May 24, 2023, the LIGO-Virgo-KAGRA researchers made improvements to the detectors, the cyberinfrastructure, and the analysis software that allow them to detect signals from further away and to extract more information about the extreme events in which the waves are generated.

Just five days after the launch of O4, things got really exciting. On May 29, 2023, the gravitational-wave signal GW230529 passed by the LIGO Livingston detector. Within minutes, the data from the detector was analysed and an alert (designated S230529ay) was released publicly announcing the signal.

Astronomers receiving the alert were informed that a neutron star and a black hole most likely merged about 650 million light-years from Earth. Unfortunately, the direction to the source could not be determined because only one gravitational-wave detector was observing at the time of the signal.

The fourth observing run is planned to last for 20 months including a couple of months break to carry out maintenance of the detectors and make a number of necessary improvements. By January 16, 2024, when the commissioning break started, a total of 81 significant signal candidates had been identified. GW230529 is the first of these to be published after detailed investigation.

The fourth observing run will resume on April 10, 2024 with the LIGO Hanford, LIGO Livingston, and Virgo detectors operating together. The run will continue until February 2025 with no further planned breaks in observing. The sensitivity of the detectors should be slightly increased after the break.

While the observing run continues, LIGO-Virgo-KAGRA researchers, including many from the University of Glasgow, are analysing the data from the first half of the run and checking the remaining 80 significant signal candidates that have already been identified.

By the end of the fourth observing run in February 2025, the total number of observed gravitational-wave signals should exceed 200.