Neutron stars and black holes are very peculiar objects, they are extremely rare in pairs. The former are the densest stable objects in the universe, remnants of stars. They are about as massive as the Sun, but only several dozen kilometers in diameter. Black holes, on the other hand, are regions of space-time with gravity so strong that even light cannot escape.
The first pair of neutron stars were observed on the Milky Way in 1974 by radio telescopes (observing so-called pulsars, or rotating neutron stars, emitting a beam of radio radiation). The existence of black pairs with stellar masses is also confirmed (confirmation was obtained a few years ago thanks to observations on gravitational waves). However, the problem was still mixed binary systems consisting of both of these bodies – such a system has not been observed in a clear way so far.
– Until now, you could observe gravitational waves that came only from the merger of two black holes or two neutron stars. The mixed system was registered for the first time, although for twenty years now, together with Professor Krzysztof Belczyński, we have anticipated such a scenario – said Professor Tomasz Bulik from the Astronomical Observatory of the University of Warsaw.
Gravitational waves picked up an unknown signal
On January 5, 2020, one of the Advanced Ligo detectors, located in Louisiana, USA, and the Advanced Virgo detector in Italy, recorded gravitational waves that came from an unknown object. The signal was emitted by a system in which a spinning neutron star and a black hole fused into a single compact object. Scientists witnessed the last few orbits before the merger. Ten days later, a signal from a similar binary was detected. The individual systems were named GW200105 and GW200115.
The scientists’ analyzes show that in GW200105 the black hole was 8.9 times the mass of the sun and the neutron star was 1.9 times the mass of the sun. They connected about 900 million years ago, and the signal of this event has now reached us. In turn, in the case of GW200115 the masses are respectively 5.7 and 1.5 times more than the mass of the Sun. Here, the objects came together nearly a billion years ago.
In both cases, the more massive objects match the range predicted by the theory of stellar evolution for black holes, and those with lower masses match the predictions for neutron stars.
The connection of a black hole with a neutron star, in addition to the signal of gravitational waves, may also result in the emission of electromagnetic radiation. The latter was not observed due to the large distance to the sources and the low accuracy of determining the position in the sky. But scientists hope to record such radiation in the future, in the event of other similar events.
An amazing combination
The described events GW200105 and GW200115 are the first reliable detections of black hole-neutron star pairing. Earlier events GW190814 and GW190426 were detected, which may also indicate similar collisions, but there is no certainty about them and scientists describe them only as candidates for pairs of objects of this type.
The research was published on Tuesday in The Astrophysical Journal Letters. The international research team included astronomers from the University of Warsaw: prof. Dorota Rosińska, prof. Tomasz Bulik, Dr. Przemysław Figura, Dr. Bartosz Idzikowski and PhD students Małgorzata Curyło, Neha Singh and Paweł Szewczyk.
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