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You wait ages for a neutron star and black hole to collide, then two pairs come along at once

Well, that was a busy month for gravitational wave boffins

Gravitational wave detectors have reportedly spotted the merger of a black hole and a neutron star – not once but twice in the same month.

This is the first time scientists have been able to confirm the detection of the merger of a black hole and neutron star, too.

Although several gravitational wave events have been detected since America's Laser Interferometer Gravitational-Wave Observatory (LIGO) made the first grav wave discovery in 2015, they have always involved collisions between objects of the same type, such as two black holes smashing into each other, or two neutron stars.

These latest discoveries, however, are the result of odd pairings: a black hole with nine times the mass of our Sun crashing into a 1.9-solar-mass neutron star, and a slightly smaller black hole measuring six solar masses impacting with a 1.5-solar-mass neutron star.

The cosmic prangs were announced on Tuesday by multiple teams working with the LIGO and Virgo detectors in Louisiana, USA, and Santo Stefano a Macerata, Italy, respectively.

As the LIGO and Virgo detectors become more and more finely tuned, the detection of these merger events are expected to crop up like buses: none for ages, and then many along at once. These latest collisions were detected ten days apart, on January 5, 2020, at 1624 UTC, and January 15 at 0423 UTC.

The first event, the more energetic of the two and involving the more massive bodies, was code-named GW200105, and took place about 900 million light-years from Earth. The second event, referred to as GW200115, was further away at about one billion light-years away.

We had long expected to see a merger between a black hole and neutron star, but there are many uncertainties that made it difficult to predict how many of this pairing there are in our universe

“We had long expected to see a merger between a black hole and neutron star, but there are many uncertainties that made it difficult to predict how many of this pairing there are in our universe,” Ryan Magee, co-author of a paper published in The Astrophysical Journal and a postdoctoral scholar at the California Institute of Technology, told The Register.

“From these two detections, we can infer that, in a fixed volume of space, there are a few more neutron star-black hole mergers per year than black hole-black hole [ones], but not as many as there are neutron star-neutron star mergers.”

Previous attempts to confirm a black hole-neutron star collision were unfruitful. One signal detected in April 2019 may have been skewed by noise, and another from August of the same year is still being debated.

Both black holes likely swallowed their neutron stars whole, leaving no trace of light for the team to follow. “These were not events where the black holes munched on the neutron stars like the cookie monster and flung bits and pieces about,” Patrick Brady, a professor at University of Wisconsin-Milwaukee and spokesperson of the LIGO Scientific Collaboration, said in a statement. “That 'flinging about' is what would produce light, and we don't think that happened in these cases.”

A merger between a galactic void and the densest type of star probably occurs within a billion light years of Earth about once a month, the team reckon.

"The detector groups at LIGO, Virgo, and KAGRA are improving their detectors in preparation for the next observing run scheduled to begin in summer 2022," Prof Brady added.

"With the improved sensitivity, we hope to detect merger waves up to once per day and to better measure the properties of black holes and super-dense matter that makes up neutron stars." ®

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