New gravitational wave signal detected
Researchers from the University 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 2023.
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 to be uncovered. Thanks to the increased sensitivity of our gravitational-wave detectors, we are now able to begin solving these mysteries.”
Researchers from the 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 are named among the authors of the new paper, reflecting the depth of our 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 two to five 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 two 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.