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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.

The first signal, picked up on Jan. 5, 2020, was traced to the merger of a black hole about nine times as massive as our sun with a neutron star that’s nearly twice the mass of the sun.

Astronomers determined that the gravitational waves came from an event that occurred 900 million light-years away. But they couldn’t identify the source of the blast, in part because LIGO’s detector in Hanford, Wash., was offline at the time. After processing clear readings from the LIGO detector in Livingston, La., and noisy readings from the Virgo detector, they could say only that the merger took place somewhere in a patch of sky that’s 34,000 times the size of a full moon.

Despite the uncertainty, the LIGO-Virgo team is confident that the readings came from an actual event rather than just a noisy detector, and that the smaller object in the crash was indeed a neutron star.

“It passes all our stringent quality checks and sticks out from all the noise events we see,” said Harald Pfeiffer, an astrophysicist at the Max Planck Institute for Gravitational Physics in Germany.

The second event, on Jan. 15, 2020, was detected by both of the LIGO observatories as well as by Virgo. It was traced to the merger of a black hole six times as massive as the sun and a 1.5-solar-mass neutron star. The collision occurred at a distance of 1 billion light-years, somewhere in an area of the sky 3,000 times as big as the full moon.

Chart of black holes and neutron stars
This graphic charts the masses of announced gravitational-wave detections and black holes and neutron stars previously constrained through electromagnetic observations. This update of the GWTC-2 O3a catalog version highlights the mergers of black holes and neutron stars known as GW200105 and GW200115. Error bars are shown on the chart. Click on the image for a larger version. (Credit: LIGO-Virgo / Frank Elavsky, Aaron Geller / Northwestern University)

When the first detection of gravitational waves from the merger of two neutron stars was made in 2017, other observatories around the world picked up a powerful flash of light. But no such flashes were detected from the two newly reported events. That’s probably because the black holes were big enough to swallow the neutron stars in one gulp.

“These were not events where the black holes munched on the neutron stars like the cookie monster, and flung bits and pieces about. That ‘flinging about’ is what would produce light, and we don’t think that happened in these cases,” said Patrick Brady, a professor at the University of Wisconsin at Milwaukee who serves as spokesperson of the LIGO Scientific Collaboration.

Based on the timing of the two detections, researchers now estimate that a merger of a black hole and a neutron star occurs within a billion light-years of Earth every month or so.

LIGO (which is short for the Laser Interferometer Gravitational-Wave Observatory) and Virgo use high-precision arrangements of lasers and mirrors to detect the ever-so-slight perturbations in the fabric of spacetime that are caused by bursts of gravitational waves. Yet another detector in Japan, known as KAGRA, joined the hunt in February 2020, just after the two newly reported events were picked up.

Although KAGRA missed out this time around, all three teams of gravitational-wave hunters should get plenty of opportunities in the years ahead to make further discoveries.

“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,” Brady said. “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.”

The latest discoveries from the LIGO-Virgo-KAGRA teams will be the subject of a webinar geared for a scientific audience at 7 a.m. PT July 1. Here’s how to register for the Zoom webinar.

By Alan Boyle

Mastermind of Cosmic Log, contributing editor at GeekWire, author of "The Case for Pluto: How a Little Planet Made a Big Difference," president of the Council for the Advancement of Science Writing. Check out "About Alan Boyle" for more fun facts.

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