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Cosmic Space

Confirmed! Black holes and neutron stars collide

Gravitational-wave astronomers are confident that they’ve filled out their repertoire of cataclysmic collisions, thanks to the detection of two cosmic crashes that each involved a black hole and a neutron star.

Over the past five years, astronomers have used the twin LIGO gravitational-wave detectors in Washington state and Louisiana, plus the Virgo detector in Italy, to pick up signals from more than 50 violent mergers of black holes with black holes, or neutron stars with neutron stars.

In 2019, the astronomers picked up readings from two events that might have been caused by hot black-hole-on-neutron-star action. But one of those detections, on April 26, 2019, could plausibly have been nothing more than noise in the detectors. The other event, on Aug. 14, 2019, involved a crash between a black hole and an object that was either the heaviest known neutron star or the lightest known black hole. The gravitational-wave hunters couldn’t say definitively which.

In contrast, astronomers leave little doubt that the gravitational waves sparked by two separate events in January 2020 were thrown off by the merger of a black hole and a neutron star. They lay out their evidence in a paper published today by The Astrophysical Journal Letters.

“With this new discovery of neutron star-black hole mergers outside our galaxy, we have found the missing type of binary. We can finally begin to understand how many of these systems exist, how often they merge, and why we have not yet seen examples in the Milky Way,” Astrid Lamberts, a member of the Virgo collaboration who works at the Observatoire de la Côte d’Azur in France, said in a news release.

There’s still some mystery surrounding the detections.

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Japan joins the global gravitational wave hunt

KAGRA detector
An illustration provides a cutaway view of the underground KAGRA gravitational-wave detector in Japan. (ICRR / Univ. of Tokyo Illustration)

Japan’s Kamioka Gravitational-Wave Detector, or KAGRA, is due to start teaming up with similar detectors in Washington state, Louisiana and Italy in December, boosting scientists’ ability to triangulate on the origins of cataclysmic cosmic events such as black hole smash-ups.

Representatives of KAGRA, the U.S.-based Laser Interferometer Gravitational-Wave Observatory (LIGO) and Europe’s Virgo detector signed a memorandum of agreement today in Toyama, Japan, to confirm their collaboration. The agreement includes plans for joint observations and data sharing.

“This is a great example of international scientific cooperation,” Caltech’s David Reitze, executive director of the LIGO Laboratory, said in a news release. “Having KAGRA join our network of gravitational-wave observatories will significantly enhance the science in the coming decade.”

Nobel-winning physicist Takaaki Kajita, principal investigator of the KAGRA project, said “we are looking forward to joining the network of gravitational-wave observations later this year.”

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