The merger and merger of a lower mass gap black hole (dark gray surface) with a neutron star with colors ranging from dark blue (60 grams per cubic centimeter) to white (600 kilograms per cubic centimeter) illustrates the strong deformations of the deep-dense material of the neutron star. Photo credits: I. Markin (University of Potsdam), T. Dietrich (University of Potsdam and Max Planck Institute for Gravitational Physics), H. Pfeiffer, A. Buonanno (Max Planck Institute for Gravitational Physics).
Scientists have detected a gravitational wave from a collision between a Neutron star and a potential black hole in the mass gap, suggesting that such cosmic events are more common than expected.
Researcher of the University of PortsmouthThe Institute of Cosmology and Gravitation (ICG) has helped discover a remarkable gravitational wave signal that could hold the key to solving a cosmic mystery.
The discovery comes from the latest results published on April 5 by the LIGO-Virgo-KAGRA collaboration, involving more than 1,600 scientists from around the world, including members of the ICG, working on the discovery Gravitational waves and use them to explore the fundamentals of science.
In May 2023, shortly after the start of the fourth LIGO-Virgo-KAGRA observation run, the LIGO-Livingston detector in Louisiana, USA, observed a gravitational wave signal from the collision of a presumably neutron star with a compact object that is 2.5 to 4, 5 times the mass of our sun.
Revealing cosmic phenomena
Neutron stars and black holes are both compact objects, the dense remnants of massive stellar explosions. What makes this signal, called GW230529, so fascinating is the mass of the heavier object. It lies within a possible mass gap between the heaviest known neutron stars and the lightest black holes. The gravitational wave signal alone cannot reveal the nature of this object. Future discoveries of similar events, particularly those associated with bursts of electromagnetic radiation, could help solve this problem.
“This discovery, the first of our exciting results from the fourth LIGO-Virgo-KAGRA observation run, shows that there may be a higher rate of similar collisions between neutron stars and low-mass black holes than we previously thought,” says Dr. Jess McIver, assistant professor at the University of British Columbia and deputy spokesperson for the LIGO Scientific Collaboration.
Since this event was only observed by a gravitational wave detector, it becomes more difficult to judge whether it is real or not.
This image shows the merger of a lower mass gap black hole (dark gray surface) with a neutron star, whose colors range from dark orange (1 million tons per cubic centimeter) to white (600 million tons per cubic centimeter). The gravitational wave signal is represented with a range of voltage amplitude values with plus polarization in colors from dark blue to cyan. Photo credits: I. Markin (University of Potsdam), T. Dietrich (University of Potsdam and Max Planck Institute for Gravitational Physics), H. Pfeiffer, A. Buonanno (Max Planck Institute for Gravitational Physics).
Advances in detection techniques
Dr. Gareth Cabourn Davies, a research software engineer at ICG, developed the tools to search for events in a single detector. He said: “Confirming events by observing them in multiple detectors is one of our most powerful tools for separating signals from noise.” By using appropriate models of background noise, we can assess an event even if we have no other detector to do so , what we saw substantiates.”
Before the discovery of gravitational waves in 2015, the masses of stellar-mass black holes were determined primarily using X-ray observations, while the masses of neutron stars were determined using radio observations. The resulting measurements fell into two distinct regions with a gap between them that was about 2 to 5 times the mass of our Sun. Over the years, a small number of measurements have widened the mass gap, which remains hotly debated among astrophysicists.
Implications of recent findings
Analysis of the signal GW230529 shows that it came from the merger of two compact objects, one with a mass between 1.2 and 2.0 times the mass of our Sun and the other just over twice as massive.
While the gravitational wave signal does not provide enough information to determine with certainty whether these compact objects are neutron stars or black holes, it is likely that the lighter object is a neutron star and the heavier object is a black one hole acts. Scientists in the LIGO-Virgo-KAGRA collaboration are confident that the heavier object is within the mass gap.
Gravitational wave observations have now provided almost 200 measurements of the masses of compact objects. Of these, only one other merger may have involved a compact object with a mass gap – the signal GW190814 came from the merger of a black hole with a compact object that exceeds the mass of the heaviest known neutron stars and may lie within the mass gap.
“While previous evidence for mass gap objects has been reported in both gravitational waves and electromagnetic waves, this system is particularly exciting because it is the first gravitational wave detection of a mass gap object paired with a neutron star,” says Dr. Sylvia Biscoveanu Northwestern University. “The observation of this system has important implications for both theories of binary evolution and electromagnetic counterparts of compact object mergers.”
Current and future observations
The fourth observation run is scheduled to last 20 months, including a break of a few months to maintain the detectors and make a number of necessary improvements. A total of 81 significant signal candidates have been identified as of January 16, 2024, when the current pause began. GW230529 is the first of these to be released after extensive investigation.
The fourth observation run will continue on April 10, 2024 with the joint operation of the LIGO Hanford, LIGO Livingston and Virgo detectors. The run will last until February 2025, with no further observation breaks planned.
As the observation run continues, LIGO-Virgo-KAGRA researchers are analyzing data from the first half of the run and reviewing the remaining 80 significant signal candidates that have already been identified. By the end of the fourth observation run in February 2025, the total number of observed gravitational wave signals is expected to exceed 200.