Gravitational waves reveal neutron star merging with mystery object in ‘mass gap’

Gravitational wave observations have revealed for the first time a merger between a neutron stare and an enigmatic object that is in the "weight gap."

Illustration of a neutron star. (Northwestern university, Kevin Gill)

Northwestern University announced last week that an international team of astrophysicists has used gravitational waves to discover for the first time a merger between a potential neutron star and a mysterious object in the “mass gap.”

The “mass gap” is the range between the heaviest known neutron star and the lightest known black home and that is where the mass of the mystery object puts it. It is between 2.5 to 4.5 times the mass of our Sun and is located about 650 million light-years away from our planet but we cannot find out what it is just using gravitational wave signals.

“While previous evidence for mass-gap objects has been reported both in gravitational and electromagnetic waves, this system is especially exciting because it’s the first gravitational-wave detection of a mass-gap object paired with a neutron star. The observation of this system has important implications for both theories of binary evolution and electromagnetic counterparts to compact-object mergers,” said Sylvia Biscoveanu, one of the researchers, in a press statement.

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GW230529, the official label for the merger, was detected by the LIGO-Virgo-Kagra collaboration in May 2023. Astrophysicists analysed the signal and found that it came from a merger of two objects — one that is between 1.2 times to 2 times the mass of our Sun and another, which is the mystery object.

Researchers determined the lighter object is most likely a neutron star and that the more massive one is potentially a black hole. However, the researchers are also confident that it is within the mass gap. If both things are true, the enigmatic object could be the lightest black hole we have ever discovered.

Gravitational wave studies have provided 200 measurements of compact-object masses. But among these, only one other merger is likely to have come from a mass-gap compact object. The signal GW190814 seems to have come from the merger of a black hole with a compact object that exceeds the mass of the heaviest known neutron stars.