Scientists have detected gravitational waves from neutron stars colliding with potential black holes in mass gaps, suggesting that such cosmic events may be more common than expected. Researchers at the University of Portsmouth's Institute for Cosmology and Gravitational (ICG) have helped detect an extraordinary gravitational wave signal that could be the key to solving the mystery of the universe. The LIGO-Virgo-Kagra Collaborative is made up of more than 1,600 scientists from around the world, including members of the ICG, to detect gravitational waves and use them to explore the fundamentals of science.
The condensation and merging of low-mass interstitial black holes (dark gray surfaces) with neutron stars, ranging in color from dark blue (60 grams per cubic centimeter) to white (600 kilograms per cubic centimeter), highlights the strong deformation of the neutron star's low-density material. Source: 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).
In May 2023, shortly after the start of the fourth LIGO-Virgo-KAGRA observational run, the LIGO Livingston probe in Louisiana, USA, observed a gravitational wave signal from what is likely to be a neutron star colliding with a compact object 2.5 to 4.5 times the mass of the Sun.
Neutron stars and black holes are compact objects, dense remnants of massive stars after explosions. This signal called GW230529 is fascinating because of its large quality. It is within the possible mass gap between the heaviest known neutron star and the lightest black hole. The gravitational wave signal alone does not reveal the nature of this celestial object. Future detection of similar events, especially those accompanied by bursts of electromagnetic radiation, may help to address this issue.
Dr. Jess McKeever, Assistant Professor at the University of British Columbia and Associate Spokesperson for the LIGO Scientific Collaboration. "This probe is our first exciting result from the fourth LIGO-Virgo-KAGRA observation run, and it reveals that the incidence of similar collisions between neutron stars and low-mass black holes may be higher than we previously thought," Jess McIver said. "
Since only one gravitational wave detector saw the event, it became more difficult to assess whether it was real or not.
This image shows the merging of a low-mass gap black hole (dark gray surface) with a neutron star, ranging in color from dark orange (1 million tons per cubic centimeter) to white (600 million tons per cubic centimeter). The gravitational wave signal is represented by a set of positively polarized strain amplitude values, ranging in color from dark blue to cyan. Source: 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 technology
Gareth Cabourn Davies, Ph.D., Research Software Engineer at ICG, has developed a tool for searching for events in a single detector. "Substantiating an event by seeing it in multiple detectors is one of our most powerful tools for separating a signal from noise," he said. By using a proper background noise model, we can judge an event even when there are no other detectors to support what we are seeing."
Before gravitational waves were detected in 2015, the mass of stellar mass black holes was mainly discovered by X-ray observations, while the mass of neutron stars was discovered by radio observations. The resulting measurements are divided into two distinct ranges, with a difference of about 2 to 5 times the mass of the Sun. Over the years, a small number of measurements have eaten away at this quality gap, which is still highly debated by astrophysicists.
Impact of the latest research findings
Analysis of the GW230529 signal revealed that it came from the merger of two compact objects, one of which had a mass of 1.2 to 2.0 times the mass of the Sun and the other had a little more than twice the mass of the Sun.
While gravitational wave signals do not provide enough information to determine whether these compact objects are neutron stars or black holes, the lighter-looking objects are likely to be neutron stars, while the heavier ones are black holes. Scientists from the LIGO-Virgo-Kagra Collaboration are convinced that heavier objects are within the mass gap.
Gravitational-wave observations have now provided measurements of the masses of nearly 200 compact objects. Of these, only one merger may involve a mass divide compact object – GW190814 signal comes from the merger of a black hole with a compact object that has a mass greater than the heaviest known neutron star and may be within the mass gap.
Dr. Sylvia Biscoveanu from Northwestern University in the United States said: "While evidence of the presence of mass gap objects in gravitational and electromagnetic waves has been previously reported, this system is particularly exciting because it is the first time that gravitational waves have detected a mass gap object paired with a neutron star. The observation of this system is of great significance for both the theory of binary star evolution and the theory of electromagnetic correspondence for the merger of compact objects."
Ongoing and future observations
The fourth observation operation is scheduled to last 20 months, including a few months of intervals, to allow for maintenance of the probe and some necessary improvements. As of January 16, 2024, which is the beginning of the current hiatus, a total of 81 important candidate signals have been identified. GW230529 is the first candidate signal to be published after a detailed investigation.
The fourth observational run will resume on April 10, 2024, with the LIGO Hanford, LIGO Livingston, and Virgo probes operating simultaneously. The observation run will continue until February 2025 with no further plans to interrupt the observations.
While the observational run continues, researchers at LIGO-Virgo-KAGRA are analyzing data from the first half of the run and examining the remaining 80 important candidate signals that have been identified. By the end of the fourth observation run in February 2025, the total number of observed gravitational wave signals will exceed 200.
编译来源：ScitechDaily