Very rare cosmic object detected in a gravitational-wave signal

In an exciting announcement from the LIGO-Virgo-KAGRA collaboration, researchers have detected a remarkable gravitational-wave signal that could hold the key to solving a cosmic mystery.

This discovery, made by a team of over 1,600 scientists from around the world, including members of the University of Portsmouth’s Institute of Cosmology and Gravitation (ICG), has the potential to shed light on the nature of compact objects and the mass gap between neutron stars and black holes.

Intriguing gravitational-wave signal: GW230529

The signal, named GW230529, was observed by the LIGO Livingston detector in Louisiana, U.S., in May 2023, shortly after the start of the fourth LIGO–Virgo–KAGRA observing run.

The gravitational waves originated 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.

The mass of the heavier object is particularly intriguing as it falls within a possible mass gap between the heaviest known neutron stars and the lightest black holes.

Dr. Jess McIver, Assistant Professor at the University of British Columbia and Deputy Spokesperson of the LIGO Scientific Collaboration, explains.

“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,” McIver said.

Challenges in assessing single-detector events

As GW230529 was only detected by one gravitational-wave detector, assessing its authenticity becomes more challenging.

Dr. Gareth Cabourn Davies, a Research Software Engineer in the ICG, developed the tools used to search for events in a single detector.

“Corroborating events by seeing them in multiple detectors is one of our most powerful tools in separating signals from noise. By using appropriate models of the background noise, we can judge an event even when we don’t have another detector to back up what we have seen,” Davies explained.

Mass gap mystery

Prior to the detection of gravitational waves in 2015, the masses of stellar-mass black holes and neutron stars were primarily determined using X-ray and radio observations, respectively.

These measurements fell into two distinct ranges with a gap between them, known as the mass gap, which spans from about two to five times the mass of our sun. In recent years, a small number of measurements have encroached on this mass gap, sparking debates among astrophysicists.

Analysis of GW230529 reveals that it originated from the merger of two compact objects, with the lighter object having a mass between 1.2 to 2.0 times that of our sun and the heavier object being slightly more than twice as massive.

Black hole within the mass gap?

While the gravitational-wave signal alone cannot determine with certainty whether these objects are neutron stars or black holes, it is likely that the lighter object is a neutron star and the heavier object a black hole within the mass gap.

Dr. Sylvia Biscoveanu from Northwestern University emphasizes the significance of this discovery.

“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,” Biscoveanu said.

“The observation of this system has important implications for both theories of binary evolution and electromagnetic counterparts to compact-object mergers,” she concluded.

Future of gravitational-wave astronomy

The fourth observing run, planned to last for 20 months with a couple of months break for maintenance and improvements, has already identified a total of 81 significant signal candidates as of January 16, 2024. GW230529 is the first of these to be published after detailed investigation.

The observing run will resume on April 10, 2024, with the LIGO Hanford, LIGO Livingston, and Virgo detectors operating together until February 2025.

As the collaboration continues to analyze the data from the first half of the run and investigate the remaining 80 significant signal candidates, the total number of observed gravitational-wave signals is expected to exceed 200 by the end of the fourth observing run.

Unraveling the mysteries of the universe with gravitational waves

The detection of GW230529 by the LIGO-Virgo-KAGRA collaboration marks a significant milestone in gravitational-wave astronomy, offering a tantalizing glimpse into the mysteries of the cosmic mass gap.

As the fourth observing run continues, with the promise of detecting over 200 gravitational-wave signals by February 2025, scientists eagerly anticipate the discovery of more events similar to GW230529.

These observations will shed light on the nature of compact objects and the mass gap between neutron stars and black holes while contributing to our understanding of binary evolution and the electromagnetic counterparts to compact-object mergers.

The tireless efforts of the global scientific community in detecting and analyzing gravitational waves bring us closer to unraveling the secrets of the universe, one signal at a time.

The full study was published here.

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