‘Extremely exciting’ signal coming from huge mystery object in outer space

Scientists have received an “extremely exciting” gravitational-wave signal from the distant universe.

The signal, found by scientists who detected ripples in the fabric of spacetime, is believed to have been caused when a neutron star merged with a mystery object. That object is remarkable to scientists because it is thought to be between 2.5 to 4.5 times the mass of our Sun – putting it in the “mass gap”.

That gap refers to the range between the heaviest known neutron star and the lightest known black hole, between two and five times the mass of our Sun. Very few objects lie in that range, and scientists know very little about the nature of them and how they might form.

Scientists do not know what the newly-discovered object, located 650 million light-years away, actually is. They hope that further examination of similar events could help clear up the mystery and explain what it is and how it might have formed.

The signal now known as GW230529 was received in May 2023. It is the first time that gravitational waves have been used to find an object that is in the mass gap pairing with a neutron star.

Researchers believe that the other, mystery object could be a black hole, but they cannot be sure. They are more confident however that it sits within that mass gap. But they said it was an “extremely exciting event” that appeared to have been caused by two compact objects, as scientists refer to dense phenomena such as black holes.

“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,” said Geraint Pratten, from the University of Birmingham.

“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.”

The discovery was made by the LIGO-Virgo-KAGRA Collaboration, which uses three different detectors spaced around the world to watch for the disruptions in spacetime that mark gravitational waves. The recent signal came towards the end of the most recent observation run last year.

That run will begin again next week, after it was paused for maintenance, and will continue into next year. Already, researchers have spotted about 80 other “significant event candidates”, and they hope that work will lead to yet more information about the mass gap and the universe more broadly.

Scientists have now used gravitational waves to get nearly 200 measurements of compact-object masses. Only one other merger has been found that is thought to have included a mass-gap compact object – the signal known as GW190814, which was reported in 2020 and is thought to have come from a black hole merging with an object that is heavier than the heaviest-known neutron stars.

Before the first detections of gravitational waves, reported early in 2016, scientists were only able to infer the nature of compact objects such as black holes and neutron stars using electromagnetic observations. That led to the idea of the “mass gap”.

“The idea of a gap between neutron-star and black-hole masses, an idea that has been around for a quarter of a century, was driven by such electromagnetic observations,” said Michael Zevin, an astrophysicist at Northwestern University. “GW230529 is an exciting discovery because it hints at this ‘mass gap’ being less empty than astronomers previously thought, which has implications for the supernova explosions that form compact objects and for the potential light shows that ensue when a black hole rips apart a neutron star.”

The discovery is being reported at the American Physical Society’s meeting, and is waiting for peer review.