An international team of astrophysicists, including two Northwestern researchers, detected the first black hole-neutron star mergers, according to a Wednesday news release.

The researchers detected two separate collisions between a black hole and a neutron star. The two new gravitational-wave events, which occurred 10 days apart in January 2020, are the first “confident observations” of the kind. According to the release, researchers can now estimate the frequency of these events in the universe. 

“The mixed collision of a black hole with a neutron star has been the elusive missing piece of the family picture of compact object mergers,” Chase Kimball, a NU graduate student who co-authored the study, said in the release. “Completing this picture is crucial to constraining the host of astrophysical models of compact object formation and binary evolution.

While the researchers are not able to detect all events, they expect around one merger per month within a distance of one billion light-years, the release said. 

Vicky Kalogera, director of the Center for Interdisciplinary Exploration and Research in Astrophysics, said  the merger may have involved the “most massive neutron star known.”

Currently, the team is preparing for its next observation run, scheduled to begin the summer of next year. Maya Fischbach, a NASA Hubble Fellowship Program Einstein Postdoctoral Fellow, said despite the discovery, there are still many unknowns. 

“We’ve now seen the first examples of black holes merging with neutron stars, so we know that they’re out there,” Fishbach said in the release. “But there’s still so much we don’t know — how small or big they can get, how fast they can spin, how they pair off into merger partners. With future gravitational wave data, we will have the statistics to answer these questions, and ultimately learn how the most extreme objects in our universe are made.”

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