LIGO–Virgo–KAGRA finds elusive mergers of black holes with neutron stars

For the first time, researchers have confirmed the detection of a collision between a black hole and a neutron star. In fact, the scientists detected not one but two such events occurring just 10 days apart in January 2020. The extreme events made splashes in space that sent gravitational waves rippling across at least 900 million light-years to reach Earth. In each case, the neutron star was likely swallowed whole by its black hole partner.

Gravitational waves are disturbances in the curvature of space-time created by massive objects in motion. During the five years since the waves were first measured, a finding that led to the 2017 Nobel Prize in Physics, researchers have identified more than 50 gravitational-wave signals from the merging of pairs of black holes and of pairs of neutron stars. Both black holes and neutron stars are the corpses of massive stars, with black holes being even more massive than neutron stars.

Now, in a new study, scientists have announced the detection of gravitational waves from two rare events, each involving the collision of a black hole and a neutron star. The gravitational waves were detected by the National Science Foundation's (NSF) Laser Interferometer Gravitational-wave Observatory (LIGO) in the United States and by the Virgo detector in Italy. The KAGRA detector in Japan joined the LIGO–Virgo network in 2020 but was not online during these detections.

"We suspected that these systems existed, but they had eluded us until now," says Ryan Magee, a postdoctoral scholar at Caltech who helped in analyzing the LIGO signals. "Continued observations of these binaries will reveal their formation channels and could one day help us understand how matter behaves at the extreme densities these processes probe."

The first merger, detected on January 5, 2020, involved a black hole about nine times the mass of our sun, or 9 solar masses, and a 1.9-solar-mass neutron star. The second merger was detected on January 15, and involved a 6-solar-mass black hole and a 1.5-solar-mass neutron star. The results were published today, June 29, in The Astrophysical Journal Letters.

Astronomers have spent decades searching for neutron stars orbiting black holes in the Milky Way, our home galaxy, but have found none so far. "With this new discovery of neutron star–black hole mergers outside our galaxy, we have found the missing 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," says Astrid Lamberts, CNRS researcher of the Virgo collaboration at Artemis and Lagrange laboratories, in Nice, France, and formerly a Caltech postdoctoral scholar.

The first of the two events, GW200105, was observed by the LIGO Livingston and Virgo detectors. It produced a strong signal in the LIGO detector but had a small signal-to-noise detection by Virgo. The other LIGO detector, located in Hanford, Washington, was temporarily offline. Given the nature of the gravitational waves, the team inferred that the signal was caused by a black hole colliding with a compact object, later identified as a neutron star. This merger took place 900 million light-years away.

"Even though we see a strong signal in only one detector, we are certain that it is real and not just detector noise. It passes all our stringent quality checks and sticks out from all noise events we see in the third observing run," says Harald Pfeiffer, group leader in the Astrophysical and Cosmological Relativity department at the Max Planck Institute for Gravitational Physics in Potsdam, Germany, and formerly a postdoctoral scholar at Caltech.

Because the signal was strong in only one detector, the location of the merger on the sky remains uncertain, lying somewhere in an area that is 34,000 times the size of a full moon.

"While the gravitational waves alone don't reveal the structure of the lighter object, we can infer its maximum mass. By combining this information with theoretical predictions of expected neutron star masses in such a binary system, we conclude that a neutron star is the most likely explanation," says Bhooshan Gadre, a postdoctoral researcher at the Max Planck Institute.

The second event, GW200115, was detected by both LIGO detectors and the Virgo detector. GW200115 comes from the merger of a black hole with a neutron star that took place roughly 1 billion light-years from Earth. Using information from all three instruments, scientists were better able to narrow down the part of the sky where this event occurred. Nevertheless, the localized area is almost 3,000 times the size of a full moon.

Astronomers were alerted to both events soon after they were detected in gravitational waves and subsequently searched the skies for associated flashes of light. None were found, but this is not surprising due to the very large distance between Earth and these mergers, which means that any light coming from them would be very dim and hard to detect with even the most powerful telescopes. Previously, in 2017, astronomers detected light from a collision between two neutron stars, first spotted in gravitational waves.

Additionally, researchers think that these neutron star–black holes mergers did not give off a light show because their black holes are big enough that they likely swallowed the neutron stars whole.

"These were not events where the black holes munched on the 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," says Patrick Brady, a professor at the University of Wisconsin-Milwaukee and spokesperson for the LIGO Scientific Collaboration.

Previously, the LIGO–Virgo network found two other candidate neutron star–black hole mergers. One event called GW190814, detected on August 14, 2019, involved a collision of a 23-solar-mass black hole with an object of about 2.6 solar masses, which could be either the heaviest known neutron star or the lightest known black hole. Another candidate event, called GW190426, and detected on April 26, 2019, was thought to possibly be a neutron star–black hole merger, but researchers have since concluded that it was more likely the result of detector noise.

Having confidently observed two examples of gravitational waves from black holes merging with neutron stars, researchers now estimate that within 1 billion light-years of Earth roughly one such merger happens per month.

"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," says Brady. "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."

Additional information about the gravitational-wave observatories:

This research is supported by NSF's LIGO Laboratory, which is a major facility funded by the NSF. LIGO is operated by Caltech and MIT, which conceived of LIGO and led the Advanced LIGO detector project. Financial support for the Advanced LIGO project was principally from the NSF with Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council), and Australia (Australian Research Council-OzGrav) making significant commitments and contributions to the project. Approximately 1,400 scientists from around the world participate in the effort to analyze the data and develop detector designs through the LIGO Scientific Collaboration, which includes the GEO Collaboration. A list of additional partners is available at my.ligo.org/census.php.

The Virgo detector is located near Pisa, Italy. The Virgo Collaboration is currently composed of approximately 650 members from 119 institutions in 14 different countries including Belgium, France, Germany, Hungary, Italy, the Netherlands, Poland, and Spain. The European Gravitational Observatory (EGO) hosts the Virgo detector and is funded by the Centre national de la recherche scientifique (CNRS) in France, the Istituto Nazionale di Fisica Nucleare (INFN) in Italy, and Nikhef in the Netherlands. A list of the Virgo Collaboration groups and more information can be found at www.virgo-gw.eu.

The KAGRA is located in Kamioka, Gifu, Japan. The host institute is the Institute of Cosmic Ray Research (ICRR) at the University of Tokyo, and the project is co-hosted by the National Astronomical Observatory of Japan (NAOJ) and the High Energy Accelerator Research Organization (KEK). KAGRA completed its construction in 2019 and later joined the international gravitational-wave network of LIGO and Virgo. The actual data-taking started in February 2020 during the final stage of the run called "O3b." KAGRA Scientific Congress is composed of more than 460 members from 115 institutes in 14 countries/regions. The list of researchers is available from gwwiki.icrr.u-tokyo.ac.jp/JGWwiki/KAGRA/KSC/Researchers. KAGRA information is at the website gwcenter.icrr.u-tokyo.ac.jp/en/.