Black Holes Swallow Neutron Stars in a Single Bite, New Results Suggest

For the first time, scientists have without a doubt observed not one but two collisions between black holes and neutron stars. These two separate mergers occurred 10 days apart in January 2020 and were seen by the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo facilities, which detect invisible gravitational waves.

The achievement marks the long-awaited completion of a trifecta of events observed by gravitational-wave interferometers: black hole–black hole collisions, neutron star–neutron star collisions and now, at last, black hole–neutron star collisions. Although the LIGO-Virgo collaboration had previously identified two candidates for this type of merger in 2019, lingering uncertainties about those events precluded any definitive discovery claim. This time, however, the telltale signatures of black holes feasting on neutron stars were unmistakable.

“It wasn’t that surprising, but it was just like, ‘Finally, it’s there,’” says Zsuzsa Márka, a Columbia University astrophysicist, LIGO collaborator, who was a co-author of the study announcing the discovery. The paper was published on June 29 in the Astrophysical Journal Letters.

The 2020 collisions each occurred independently in distinct, widely separated regions of the sky and at astronomically vast distances from Earth. One, on January 5, involved a black hole with a mass nearly nine times greater than that of the sun and a neutron star that was almost twice as massive as our star. The other, on January 15, involved a black hole of 5.7 solar masses and a neutron star packing one and a half times our sun’s heft. Based on the short period in which both collisions took place, physicists now estimate that a merger between a black hole and neutron star occurs approximately once a month somewhere within a billion light-years of the solar system.

Albert Einstein’s 1916 prediction of gravitational waves, or ripples in spacetime that can be caused by the motions of extremely massive objects, has borne fruit for physicists since 2015. In September of that year, LIGO detected gravitational waves from the collision of two black holes. Subsequently, LIGO’s capabilities were upgraded, and Italy’s Virgo and Japan’s Kamioka Gravitational-Wave Detector (KAGRA) joined in the detection of gravitational waves—leading to more observations of binary black hole mergers and the first detection of a binary neutron star collision in 2017. In a way, the observation of a neutron star coalescing with a black hole “completes our collection,” says Chase Kimball, an astrophysics graduate student at Northwestern University and a co-author of the research.