Astronomers using NSF’s Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States and by the Virgo detector in Italy have observed gravitational-wave signals from two compact binary inspirals that are consistent with neutron star-black hole binaries. The two events occurred at least 900 million light-years away; in each case, the neutron star was likely swallowed whole by its black hole partner.
An artist’s impression of a black hole-neutron star merger. Image credit: Carl Knox, OzGrav & Swinburne University.
“With this new discovery of neutron star-black hole mergers outside our Milky Way Galaxy, we have found the missing type of binary,” said Dr. Astrid Lamberts, an astronomer at Observatoire de la Côte d’Azur and CNRS.
“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.”
The first of the two gravitational-wave events, dubbed GW200105, was detected on January 5, 2020.
It produced a strong signal in one of the two LIGO detectors but had a small signal-to-noise in the Virgo detector. The other LIGO detector was temporarily offline.
Given the nature of the gravitational waves, the astronomers inferred that the signal was caused by a black hole about 9 times the mass of our Sun colliding with a 1.9-solar-mass 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 conclude that it is real and not just detector noise,” said Dr. Harald Pfeiffer, an astronomer at the Max Planck Institute for Gravitational Physics.
“It passes all our stringent quality checks and sticks out from all noise events we see in the third observing run.”
“While the gravitational waves alone don’t reveal the structure of the lighter object, we can infer its maximum mass,” added Dr. Bhooshan Gadre, also from the Max Planck Institute for Gravitational Physics.
“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.”
The second merger, GW200115, was detected on January 15, 2020, and involved a 6-solar-mass black hole and a 1.5-solar-mass neutron star.
It was detected by both LIGO detectors and the Virgo detector and took place roughly 1 billion light-years from Earth.
“These were not events where the black holes munched on the neutron stars like the cookie monster and flung bits and pieces about,” said Professor Patrick Brady, an astronomer at the University of Wisconsin-Milwaukee and spokesperson of the LIGO Collaboration.
“That ‘flinging about’ is what would produce light, and we don’t think that happened in these cases.”
“The events occurred about a billion years ago but were so massive that we are still able to observe their gravitational waves today,” said Professor Susan Scott, an astronomer at the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) and the Australian National University.
“These collisions have shaken the Universe to its core and we’ve detected the ripples they have sent hurtling through the cosmos.”
“Each collision isn’t just the coming together of two massive and dense objects. It’s really like Pac-Man, with a black hole swallowing its companion neutron star whole.”
“These are remarkable events and we have waited a very long time to witness them. So it’s incredible to finally capture them.”
The results appear in the Astrophysical Journal Letters.
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R. Abbott et al. 2021. Observation of Gravitational Waves from Two Neutron Star-Black Hole Coalescences. ApJL 915, L5; doi: 10.3847/2041-8213/ac082e
