Unusually Lightweight Black Hole Candidate Spotted by LIGO

In May 2023, shortly after LIGO (Laser Interferometer Gravitational-wave Observatory) turned back on for its fourth run of observations, it detected a gravitational-wave signal from the collision of an object, most likely a neutron star, with a suspected black hole possessing a mass that is 2.5 to 4.5 times more than that of our Sun. This signal, called GW230529, is intriguing to researchers because the candidate black hole's mass falls within a so-called mass gap between the heaviest known neutron stars, which are slightly more than two solar masses, and the lightest known black holes, which are about five solar masses. While the gravitational-wave signal alone cannot reveal the true nature of this object, future detections of similar events, especially those accompanied by bursts of light, could hold the key to answering the question of how lightweight black holes can be.

"The latest finding demonstrates the impressive science capability of the gravitational-wave detector network, which is significantly more sensitive than it was in the third observing run," says Jenne Driggers (PhD '15), detection lead scientist at LIGO Hanford in Washington, one of two facilities, along with LIGO Livingston in Louisiana, that make up the LIGO Observatory.

LIGO made history in 2015 after carrying out the first direct detection of gravitational waves in space. Since then, LIGO and its partner detector in Europe, Virgo, have detected nearly 100 mergers between black holes, a handful between neutron stars, as well as mergers between neutron stars and black holes. The Japanese detector KAGRA joined the gravitational-wave network in 2019, and the team of scientists who collectively analyze data from all three detectors is known as the LIGO–Virgo–KAGRA (LVK) collaboration. The LIGO observatories are funded by the National Science Foundation (NSF), and were conceived, built, and are operated by Caltech and MIT.

The latest finding also indicates that collisions involving lightweight black holes may be more common than previously believed.

"This detection, the first of our exciting results from the fourth LIGO–Virgo–KAGRA observing run, reveals that there may be a higher rate of similar collisions between neutron stars and low mass black holes than we previously thought," says Jess McIver, an assistant professor at the University of British Columbia, deputy spokesperson of the LIGO Scientific Collaboration, and a former postdoctoral fellow at Caltech.

Prior to the GW230529 event, one other intriguing mass-gap candidate object had been identified. In that event, which took place in August 2019 and is known as GW190814, a compact object of 2.6 solar masses was found as part of a cosmic collision, but scientists are not sure if it was a neutron star or black hole.

After a break for maintenance and upgrades, the detectors' fourth observing run will resume on April 10, 2024, and will continue until February 2025.

The preprint GW230529 study titled, "Observation of Gravitational Waves from the Coalescence of a 2.5-4.5 M_\odot Compact Object and a Neutron Star," has been posted online.

Read the full story from the LVK collaboration.