Researchers from the Institute of Cosmology and Gravitation have helped to detect a gravitational-wave signal that could solve a key cosmic mystery.
The discovery of the gravitational-wave signal is from a set of results announced recently by the LIGO-Virgo-KAGRA collaboration, which comprises more than 1,600 scientists from around the world.
The collaboration includes members of the ICG, which aims to detect gravitational waves and use them to explore the fundamentals of science.
Observation of the gravitational-wave signal
Shortly after the start of the fourth LIGO-Virgo-KAGRA observing run in May 2023, the US LIGO Livingston detector observed a gravitational-wave signal from the collision of what is most likely a neutron star with a compact object 2.5 to 4.5 times the mass of the Sun.
Neutron stars and black holes are compact objects. What makes this signal, GW230529, interesting is the mass of the heavier object.
The object falls within a mass-gap between the heaviest known neutron stars and the lightest black holes.
The gravitational-wave signal cannot reveal the nature of this object by itself.
Therefore, future detections of similar events are required to solve this mystery.
“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,” said Dr Jess McIver, Assistant Professor at the University of British Columbia and Deputy Spokesperson of the LIGO Scientific Collaboration.
Assessing the reality of the event
As the event was only seen by one gravitational-wave detector, assessing whether it is real becomes more difficult.
Dr Gareth Cabourn Davies, a Research Software Engineer in the ICG, said: “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.”
Previous way to detect masses of black holes and neutron stars
Before gravitational waves were detected in 2015, the masses of stellar-mass black holes were found using X-ray observations. The masses of neutron stars were found using radio observations.
The measurement fell into two distinct ranges, with a gap between them from about two to five times the mass of the Sun.
Over the years, some measurements have encroached on the mass gap, which is highly debated amongst astrophysicists.
Information provided by the newly detected signal
Analysis of the gravitational-wave signal shows that it came from the merger of two compact objects. One object had a mass between 1.2 and 2.0 times that of our Sun, and the other was slightly more than twice as big.
Although the gravitational-wave signal does not provide enough information to determine with certainty whether these objects are neutron stars or black holes, the lighter object is likely a neutron star, and the heavier one is a black hole.
Scientists in the LIGO-Virgo-KAGRA Collaboration are confident that the heavier object is within the mass gap.
Gravitation-wave signals have 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.
“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,” said Dr Sylvia Biscoveanu from Northwestern University.
“The observation of this system has important implications for both theories of binary evolution and electromagnetic counterparts to compact-object mergers.”
The fourth observing run
The fourth observing run is planned to last for 20 months, including a break to make several improvements.
By 16 January 2024, when the current break started, a total of 81 significant signal candidates had been identified.
GW230529 is the first of these to be published after a detailed investigation.
The fourth run will resume on 10 April 2024 and will continue until February 2025 with no further planned breaks in observing.
While the run continues, the team will analyse the data from the first half of the run and check the remaining 80 significant signal candidates that have been identified.
By the end of the fourth observing run, the total number of observed gravitational-wave signals should exceed 200.
