Gravity waves show black hole-neutron star mergers

An artist’s impression of a compact neutron star merging with a black hole. Image: Carl Knox, OzGrav/Swinburne

On 5 January 2020, gravity waves reached Earth that were generated a billion years ago by the merger of a 1.9-solar-mass neutron star and a black hole with nine times the mass of the Sun. Ten days later, space rippled once again when another set of gravity waves swept through the solar system, indicating the merger of a six-solar-mass black hole and a neutron star with 1.5 solar masses some 900 million light years from Earth.

The observations are the first confirmed examples of mixed mergers between black holes and neutron stars.

“Gravitational waves have allowed us to detect collisions of pairs of black holes and pairs of neutron stars, but 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,” said Chase Kimball, a Northwestern University graduate student who co-authored a study in Astrophysical Journal Letters.

“Completing this picture is crucial to constraining the host of astrophysical models of compact object formation and binary evolution. Inherent to these models are their predictions of the rates that black holes and neutron stars merge amongst themselves. With these detections, we finally have measurements of the merger rates across all three categories of compact binary mergers.”

The 15 January event, catalogued GW200115, was seen by both gravity wave detectors operated by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States and by the Virgo interferometer in Italy. The 5 January merger was seen by LIGO but was too weak to be detected by VIRGO.

Neither merger generated detectable electromagnetic radiation, but the gravity wave observations indicate a statistical probability of at least one such merger per month within a billion light years of Earth.

“We’ve now seen the first examples of black holes merging with neutron stars, so we know that they’re out there,” said Maya Fishbach, a NASA Einstein Postdoctoral Fellow at Northwestern University. “But there’s still so much we don’t know about neutron stars and black holes — how small or big they can get, how fast they can spin, how they pair off into merger partners.”

Follow-on research is planned and with additional data, Fishbach said, “we will have the statistics to answer these questions, and ultimately learn how the most extreme objects in our universe are made.”