When Stars Are Swallowed by Black Holes, New Findings Suggest

For the first time, scientists have confirmed the existence of not one, but two black hole-neutron star collisions. The Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo facilities, which detect invisible gravitational waves, saw these two distinct mergers 10 days apart in January 2020.

The achievement completes a long-awaited trinity of gravitational-wave interferometer observations: black hole–black hole collisions, neutron star–neutron star collisions, and now, at long last, black hole–neutron star collisions. Even though the LIGO-Virgo team has previously discovered two possibilities for this sort of merger in 2019, the uncertainty surrounding those events prevented any definite claim of discovery. The telltale signs of black holes eating on neutron stars, on the other hand, were unmistakable this time.

Zsuzsa Márka, a Columbia University astronomer and LIGO partner who was a co-author of the research reporting the finding, says, “It wasn’t so surprising, but it was simply like, ‘Finally, it’s there.” The study was published in the Astrophysical Journal Letters on June 29.

Each of the 2020 collisions took place in a different, widely separated region of the sky, at astronomically great distances from Earth. On January 5, for example, a black hole almost nine times the mass of the sun collided with a neutron star nearly twice the mass of our sun. The other, which occurred on January 15, included a black hole with a mass of 5.7 solar masses and a neutron star with a mass of one and a half times that of our sun. Physicists currently believe that a merger between a black hole and a neutron star occurs once a month somewhere within a billion light-years of the solar system, based on the brief time during which both collisions occurred.

Since 2015, scientists have been able to confirm Albert Einstein’s 1916 prediction of gravitational waves or disturbances in spacetime generated by the movements of enormously large objects. LIGO discovered gravitational waves from the collision of two black holes in September of that year. Following that, LIGO’s capabilities were enhanced, and Italy’s Virgo and Japan’s Kamioka Gravitational-Wave Detector (KAGRA) participated in the detection of gravitational waves, resulting in further binary black hole merger observations and the first detection of a binary neutron star collision in 2017. The discovery of a neutron star merging with a black hole “completes our collection,” according to Chase Kimball, a Northwestern University astrophysics graduate student and co-author of the study.

The interferometers at LIGO, Virgo, and KAGRA each have two arms that “wiggle” slightly as a result of gravitational wave disturbances. The signals created by these wiggles—affectionately known as chirps—were particularly apparent during the two 2020 occurrences, according to Márka, notably in the case of the first merger on January 5. She continues, “It was absolutely a lovely chirping occasion.”

According to Alessandra Buonanno, director of the Max Planck Institute for Gravitational Physics in Germany, a LIGO collaborator and co-author of the June 29 study, earlier interferometer observations in April and August 2019 drew scientific and media attention as a potential black hole–neutron star mergers. However, the specifics of each of those occurrences reduced confidence in their identification, whereas the most recent indications were more conclusive. One of the objects involved in the August 2019 collision fell into the “mass gap,” a theorized region in which neither black holes nor neutron stars are expected to exist. If it turned out to be a neutron star, it would be the heaviest object ever discovered. It would be the lightest black hole ever discovered if it was a black hole. Researchers are perplexed, and they’re still disputing what they witnessed. However, because each merger is a one-time occurrence, no more information from that far-off event is likely to emerge to provide a definite response.

Astrophysicists researching these mergers frequently anticipate finding accompanying electromagnetic emissions from the event—some heavenly light sparks created in addition to gravitational waves by the cosmic cataclysm. The two 2020 sightings, however, were classified as a neutron star–black hole mergers based only on gravitational waves, rather than any electromagnetic signal, according to François Foucart, a physics professor at the University of New Hampshire who was not involved in the research.

Before the 2020 measurements, physicists were unsure if a far more massive black hole would consume the neutron star whole, like Pac-Man, or if its tidal forces would tear the star before enveloping it like a tornado ripping apart a home. In the latter situation, a pileup of hot, blazing material around the black hole would be expected, which a high-powered telescope might detect. Buonanno confirmed that neither collision produced any lights or other electromagnetic indications. Still, she adds, this does not rule out the possibility of seeing light-based counterparts in future collisions because their formation is dependent on parameters including the black hole and neutron star’s masses, velocities, orientations, and cosmic surroundings.

According to Kimball, the discovery also gets scientists one step closer to understanding how these sorts of binaries emerge. Each of the two progenitor couples may have been born together and lived out their lives as stars. Or they might have started to orbit one other later in their lives as members of a globular cluster, which has dense swarms of stars in its center. He adds that these two mergers alone do not provide all of the answers, but that demographic studies of a broader population of the black hole–neutron star collisions will eventually indicate which path is more prevalent.

According to Foucart, future studies of these mergers may provide answers to another mystery: how our cosmos became loaded with gold, platinum, and other heavier metals. He goes on to say that around half of the elements heavier than iron are created in enormous cosmic collisions or explosions and that a greater understanding of the frequency of black hole–neutron star mergers would reveal how much of the universe’s heavier elements they produce.

The LIGO and Virgo detectors are now undergoing upgrades in preparation for an observation run that will begin in June 2022. For that run, KAGRA, Japan’s detector, will go online. These enhancements will improve the detectors’ combined capacity to determine the exact location of an event in the sky, which will help astronomers in scanning the correct portions of the skies with traditional telescopes to catch electromagnetic equivalents, according to Foucart.
According to Buonanno, “seeing these neutron stars–black holes for the first time is simply the tip of the iceberg of this population.”