‘Mass-gap’ gravitational wave signal discovered by international scientists.

The LIGO-Virgo-KAGRA (LVK) Collaboration, which includes astronomers and scientists from the University of Birmingham, has detected a remarkable gravitational-wave signal called GW230529.

The discovery was made in May 2023 and is thought to have been caused by 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. What makes GW230529 so interesting to scientists is the mass of the heavier object, as it falls within a possible mass-gap between the heaviest known neutron stars and the lightest black holes.

The observation is being presented at the American Physical Society’s meeting today (5^th April) and is awaiting peer review.

Neutron stars and black holes are both compact objects, the dense remnants of massive stellar explosions. Before the detection of gravitational waves, the masses of stellar-mass black holes were found using X-rays and the masses of neutron stars were found via radio observations. These 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.

Dr Geraint Pratten, Royal Society University Research Fellow at the Institute for Gravitational Wave Astronomy, University of Birmingham, was part of the team that analysed the findings. He explained: "GW230529 is an extremely exciting event that was produced by the merger of two compact objects. Our analysis shows that the heavier object had a mass about 2.5 - 4.5 times that of the Sun whereas the lighter object was only about 1.2 - 2.0 as massive as the Sun. We could not determine with certainty if the compact objects are black holes or neutron stars, as the gravitational wave signal does not provide enough information. However, it is very likely that this was the merger between a black hole and a neutron star. Either way, we are very confident that the heavier object falls within the mass-gap. Our analysis is already providing important insights, allowing us to further refine our understanding of the astrophysical processes behind these mergers.”

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.

GW230529 is the first gravitational-wave detection of a mass-gap object paired with a neutron star and has important implications for our understanding of the formation and evolution of compact binaries, as well as possible electromagnetic counterparts to these mergers.

Dr Patricia Schmidt, Associate Professor at the Institute for Gravitational Wave Astronomy, said: “The most likely interpretation of this event is the merger of a black hole with a neutron star. As their masses are rather similar, we also expect the emission of electromagnetic radiation in such a collision. This exciting observation could significantly boost the prospects for detecting EM counterparts from neutron star – black hole mergers in the future.”

Thanks to improvements made to the gravitational wave detectors, the cyberinfrastructure and the analysis software, the LIGO-Virgo-KAGRA researchers were able to detect signals from further away and extract more information about the extreme events which caused the waves. Just five days into the fourth observing run GW230529 passed by the LIGO Livingston detector and within minutes, the data from the detector was analyzed and an alert 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.

Dr Pratten concluded: “In addition to GW230529, we have identified about 80 other significant event candidates to investigate. We expect that by February 2025, when the fourth observing run ends, we will have observed more than 200 gravitational-wave signals. Future detections of similar events, especially those accompanied by bursts of electromagnetic radiation, could hold the key to solving this cosmic mystery of mass-gap and further our understanding of the universe.”