Physicists confirm two cases of “elusive” black hole/neutron star mergers

The successful gravitational-wave detections just keep coming for the LIGO-Virgo-KAGRA collaboration, which has now confirmed two separate “mixed” mergers between black holes and neutron stars, sending powerful gravitational waves rippling across spacetime. Those signals were detected last year by the collaboration, just 10 days apart. A year and a half later, the events officially constitute the first confirmed detection of mixed mergers, as described in a new paper published in 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,” said co-author Astrid Lamberts of CNRS, a researcher on the Virgo collaboration in Nice. “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.”

LIGO detects gravitational waves via laser interferometry, using high-powered lasers to measure tiny changes in the distance between two objects positioned kilometers apart. LIGO has detectors in Hanford, Washington state, and in Livingston, Louisiana. A third detector in Italy, Advanced VIRGO, came online in 2016. In Japan, KAGRA is now online, and it’s the first gravitational-wave detector in Asia and the first to be built underground. Construction began on LIGO-India earlier this year, and physicists expect it will turn on sometime after 2025.

To date, the LIGO collaboration has detected dozens of merger events since its first Nobel Prize-winning discovery, all involving either two black holes or two neutron stars. Most recently, last year, the collaboration announced the detection of two more black hole mergers. One was the most massive and most distant black hole merger yet detected, and it produced the most energetic signal detected thus far. It showed up in the data as more of a “bang” than the usual “chirp.” The detection also marked the first direct observation of an intermediate-mass black hole.

As the detectors’ sensitivity improves, confirmed events are becoming much more frequent. In just one month after starting a new run on April 1, 2019, for example, LIGO/VIRGO had seen five gravitational-wave events, three from merging black holes and one from merging neutron stars, with the fifth possibly being the elusive black hole/neutron star merger.

Graphic for masses of announced gravitational-wave detections.
Enlarge / Graphic for masses of announced gravitational-wave detections.

LIGO-Virgo/Frank Elavsky, Aaron Geller/Northwestern

As Ars’ John Timmer previously reported, on April 26, 2019, all three detectors were online when an extremely distant event occurred roughly 1.2 billion light years away. This event (designated GW190814) was tentatively identified as the first possible merger between a 23-solar-mass black hole and a 2.6-solar-mass neutron star—the heaviest known neutron star, although alternatively, it might be the lightest known black hole. About a week later, the detectors picked up a second possible black hole/neutron star merger, although that could have been detector noise.

“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 is among the many co-authors of the new paper. “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.”

Both LIGO Livingston and Virgo picked up the signal from the first of the black hole/neutron star mergers on January 5, 2020, designated GW200105. But the signal was only really strong at the Livingston detector, while LIGO-Hanford was offline entirely at the time, so the collaboration was not able to precisely locate the position of the merger in the sky. They were only able to narrow it down to an area roughly 34,000 times the size of a full moon. However, the team was nonetheless able to deduce that the signal came from a mixed merger, involving a black hole of about nine solar masses and a neutron star of about 1.9 solar masses some 900 million light years away.

All three detectors were online when the signal from the second black hole/neutron star merger (designated GW200115) arrived on January 15, however. That event involved a black hole of six solar masses merging with a neutron star of about 1.5 solar masses, and the team was able to narrow the merger’s location to an area about 3,000 times the size of a full moon. Based on their analysis of these two events, the LIGO-Virgo researchers think that this kind of mixed merger could occur about once a month, although not all of those events will be detectable, given the current sensitivities of the various detectors.

Artistic rendition of a black hole merging with a neutron star.
Enlarge / Artistic rendition of a black hole merging with a neutron star.

“Following the tantalizing discovery, announced in June 2020, of a black-hole merger with a mystery object, which may be the most massive neutron star known, it is exciting also to have the detection of clearly identified mixed mergers, as predicted by our theoretical models for decades now,” said co-author Vicky Kalogera of Northwestern University. “Quantitatively matching the rate constraints and properties for all three population types will be a powerful way to answer the foundational questions of origins.”

As for how these mixed systems form in the first place, one possibility is that there were two stars already orbiting each other, with sufficient masses that they went supernova, leaving behind a black hole and neutron star. Alternatively, it could be that the neutron stars and black holes in such mixed mergers formed separately from supernova explosions and eventually drifted toward each other to form a binary system. This would be more likely to happen in globular clusters, which have a higher density of stars. Astrophysicists can study the direction of the black hole’s spin in a given mixed merger for clues as to which of these two mechanisms led to the merger’s formation.

“These were not events where the black holes munched on the neutron stars like the Cookie Monster and flung bits and pieces about.”

This is the era of so-called multi-messenger astronomy, so astronomers around the world in turn hunted for any telltale flashes of light in their telescopes that might be an accompanying electromagnetic signature from those mergers, to no avail. This is likely due to the fact that the mergers occurred so far away; any light produced would be much too faint for detection by the time it reached our telescopes. It’s also probable that there was no accompanying light show, because the black hole simply swallowed the neutron star whole.

“These were not events where the black holes munched on the neutron stars like the Cookie Monster and flung bits and pieces about,” said co-author Patrick Brady of the University of Wisconsin-Milwaukee, spokesperson of the LIGO Scientific Collaboration. “That ‘flinging about’ is what would produce light, and we don’t think that happened in these cases.”

There should be many more such discoveries to come, since LIGO, Virgo, and KAGRA are currently making additional improvements to their detectors in preparation for a new run next summer. With those upgrades, they estimate they could detect as much as one gravitational-wave event per day.

“We’ve now seen the first examples of black holes merging with neutron stars, so we know that they’re out there,” said co-author Maya Fishbach, also from Northwestern. “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. 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.”

DOI: The Astrophysical Journal, 2021. 10.3847/2041-8213/ac082e (About DOIs).

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