A black hole is typically a result of two neutron stars colliding quietly. In a collaborative study, scientists discovered, through a series of simulations, that creating black holes comes with a loud 'bang'.
Professor David Radice from Penn State described that strong gravitational waves are the result of two incredibly dense collapsed neutron stars that formed a black hole. The impact of the waves can be detected with the Laser Interferometer Gravitational-wave Observatory (LIGO) in the United States or Europe's Virgo interferometer.
Their study has been recently published in the Monthly Notices of the Royal Astronomical Society in collaboration with Computational Relativity (CoRe).
Scientists have previously known that the gravitational forces of black holes swallow space objects, such as stars and other smaller astrophysical objects. However, the team's simulations showed that the black hole does not always swallow any other radiation that could have come out of the merger.
Colliding Stars
There are four types of black holes in space: intermediate, miniature, stellar, and supermassive. While stellar black holes are the result of a massive star collapsing upon itself, intermediate black holes are believed to be the result of a cluster of stars colliding in a chain reaction.
Tim Roberts from the University of Durham in the U.K. described intermediate-mass black holes (IMBH) as, "there have been hints that they exist, but IMBHs have been acting like a long-lost relative that isn't interested in being found."
During the team's series of simulations using supercomputers from the National Science Foundation's XSEDE, when two colliding neutron stars have different masses, the larger star destroys the smaller one.
This results in a slower merger as an electromagnetic 'bang' escapes. Astronomers can then detect similar electromagnetic signals from Earth when the same activity is happening in space, looking for the unique signatures of the 'bang' from stars colliding.
"Recently, LIGO announced the discovery of a merger event in which the two stars have possibly very different masses," said Radice. "The main consequence in this scenario is that we expect this very characteristic electromagnetic counterpart to the gravitational wave signal."
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Detecting the Invisible
Black holes are certain 'invisible' areas in space where gravitational forces are so strong that even light cannot escape them. This is the result of dying or colliding stars.
Scientists have been able to detect black holes by studying the behavior of stars or other objects orbiting a black hole and even being swallowed by them. Also, when a black hole and a star get close enough, they create a high-energy light that can be detected with satellites and telescopes.
LIGO detected its first neutron-star merger in 2017, which created a 'kilonova' explosion. The massive collision was a result of two equally massed neutron stars, about 2.7 times the mass of the Sun. In 2019, another collision was detected, named GW190425, from two unequal stars. The larger mass seemed to be 3.5 times the size of the Sun with the smaller star half that size.
Radice said that there is still much to learn about the properties of neutron stars and how black holes form. "In order to understand them, we have to simulate many possible models to see which ones are compatible with astronomical observations."
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