Black holes and neutron stars coalesce in dense star clusters

US, WASHINGTON (ORDO NEWS) — The mergers between black holes and neutron stars in dense star clusters are completely different from those that occur in isolated regions where there are few stars. The features associated with this can be crucial for the study of gravitational waves and their source.

Dr. Manuel Arca Sedda of the Institute for Astronomical Computing at the University of Heidelberg came to this conclusion in a study using computer simulation. This study may provide a critical view of the merger of two massive stellar objects that astronomers observed in 2019. The results of the study were published in the journal Communications Physics.

Stars are much more massive than our Sun, usually ending their lives with a neutron star or a black hole. Neutron stars emit regular radiation pulses that allow them to be detected. For example, in August 2017, when the first double fusion of neutron stars was observed, scientists around the globe discovered the light from the explosion using their telescopes. Black holes, on the other hand, usually remain hidden because their gravitational attraction is so strong that even light cannot escape, making them invisible to electromagnetic detectors.

If two black holes merge, then this event may be invisible, but nevertheless it can be detected by ripples in space-time in the form of so-called gravitational waves. Some detectors, such as the Laser Interferometric Gravitational Wave Observatory (LIGO) in the United States, are capable of detecting these waves. The first successful direct observation was made in 2015. The signal was obtained as a result of the merger of two black holes. But this event may not be the only source of gravitational waves. They can also arise as a result of the fusion of two neutron stars or a black hole with a neutron star. According to Dr. Ark Sedda, detecting these differences is one of the main problems in observing these events.

In his research, he analyzed the fusion of pairs of black holes and neutron stars. He used detailed computer simulation to study the interactions between a system consisting of a star and a compact object, such as a black hole, and a third massive wandering object, which is necessary for merging. The results show that such three-body interactions can indeed contribute to the fusion of black holes and neutron stars in dense stellar regions such as globular star clusters. “You can define a special family of dynamic mergers that is very different from mergers in isolated areas,” explains Manuel Arca Sedda.

The fusion of a black hole with a neutron star was first noticed by the observatories of gravitational waves in August 2019. However, optical observatories around the world could not detect an electromagnetic twin in the area where the signal of the gravitational wave came from, and this suggested that the black hole completely absorbed the neutron star without destroying it first. If this is confirmed, then this may be the first observable fusion of a black hole and a neutron star found in a dense stellar medium, as described by Dr. Ark Sedda.


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