This signal forces us to reconsider the range of masses of compact objects
Researchers from the Institute of Cosmology and Gravity (ICG) at the University of Portsmouth, together with the LIGO-Virgo-KAGRA collaboration, which includes more than 1,600 scientists from different countries, have detected a signal of gravitational waves that may contain a clue to one of the cosmic secrets.
In May 2023, after the start of the fourth observational launch LIGO-Virgo-KAGRA, the LIGO detector in Louisiana recorded a signal from the collision of a likely neutron star and a compact object having a mass of 2.5-4.5 times more than the mass of the Sun.
What makes the signal called GW230529 interesting — is the mass of the heavier object. It is within the possible mass gap between the heaviest known neutron stars and the lightest black holes. The gravitational wave signal itself cannot reveal the nature of the object; only future detections of similar events, especially those accompanied by bursts of electromagnetic radiation, could help solve this problem.
Dr. Jess McIver, representative of the LIGO Science Collaboration, notes that this discovery, the first of the LIGO-Virgo-KAGRA cycle, indicates the possibility of collisions between neutron stars and low-mass black holes more frequently than previously thought.
Since this event was only seen by one gravitational wave detector, assessing its reality becomes more difficult. Dr Gareth Caborn Davies, a research engineer at the ICG, has developed single-detector event search tools. He noted: «Confirming events by observing them with multiple detectors — one of the best ways to separate signals from noise. But using noise models, we can judge an event even if we have no other detector to confirm the observation.
Before the detection of gravitational waves in 2015, the masses of black holes and neutron stars were determined using X-ray and radio observations, respectively. The resulting measurements fell into two separate ranges, with a mass gap between them of about two to five times the mass of our Sun. Over the years, a small number of measurements have been found in a smaller mass range and have sparked debate in the scientific community regarding the difference in mass of these objects.
Analysis of the GW230529 signal shows that it occurred as a result of the merger of two objects: one of them was 1.2-2 times more massive than the Sun, and the other — more than twice. Probably the smaller object was a neutron star, and the larger — black hole. Further research of this signal by the international community of LIGO-Virgo-KAGRA scientists will help confirm this hypothesis.
Currently, thanks to observations of gravitational waves, scientists have almost 200 measurements of the masses of compact objects. However, only one merger, known as GW190814, has attracted particular attention because it could be associated with a compact object whose mass exceeds that of the heaviest neutron stars and may be within the mass difference of
«This system is particularly interesting because it is the first time that gravitational waves associated with an object with a difference in mass are detected, paired with a neutron star», — notes Dr. Silvia Biscovanu from Northwestern University. «The observation of this system has important implications for theories of binary evolution and can serve as an analogue to electromagnetic radiation from mergers of compact objects».
The fourth cycle of observations is planned for 20 months, including breaks for maintenance of detectors and making necessary modifications. To date, 81 significant events have been identified. GW230529 — the first of them to be published after detailed analysis.
The fourth observation cycle resumed on April 10, 2024, when the LIGO Hanford, LIGO Livingston and Virgo detectors were launched simultaneously. Continuation of observations is planned until February 2025 without interruption.