A billion years ago, long before the dawn of complex life on Earth, a black hole several times more massive than the sun engulfed the collapsed core of a once-giant star.
The immense collision between two of the universe’s most extreme objects sent gravitational waves hurtling through the cosmos, like ripples on an enormous pond.
In January 2020, those ancient waves reached the shores of our solar system, where they were picked up by ultrasensitive equipment at observatories in the United States and Italy. It marked the first time astronomers had ever detected a black hole swallowing a neutron star.
“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 was detected on Jan. 15, 2020. It involved a black hole six times the mass of the sun and a neutron star about 1½ times the sun’s mass.
In both cases, the neutron stars were consumed by their companions, according to the study. There were no associated flashes of light — and that was no surprise. There probably wasn’t any light show to see, researchers said, because the black holes were big enough to completely swallow the neutron stars.
“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 study was published in Astrophysical Journal Letters.