Scientists catch a glimpse of a black hole swallowing a neutron star.

After more than four years of exploring a menagerie of cosmic events through gravitational waves, scientists have finally spotted the third expected strain of collision - two times.

The new flavor of the collision consists of a black hole and a neutron star, which makes it a sort of mashup. The scientists observed tens of merging pairs of black holes, and some merging pairs of neutron stars, superdense stellar bodies. But an accident between a black hole and a neutron star, although foreseen by scientists, had not been definitively detected.

Now, the researchers say they did exactly that, observing the unique undulations in space-time caused by such a collision.

"With this discovery of neutron star-black hole mergers outside our galaxy, we have found the missing type of binary," Astrid Lamberts, a CNRS researcher at Observatoire de la Côte d'Azur in France, said in a statement. "We can finally begin to understand how many of these systems exist, how often they merge, and why we have yet to see examples from the Milky Way.”

The two new detections occurred in January 2020, with only a 10-day interval, and the collisions are now known as GW200105 and GW200115 for the dates they were detected. One was detected by the twin detectors Laser Interferometer Gravitational-wave Observatory (LIGO) and the similar Virgo detector in Europe, the other by a single detector LIGO and Virgo. (The partnership now includes a detector in Japan called KAGRA, but this facility only started observing in February 2020.)

GW200115 was specifically detected and observed by the three facilities. Scientists believe that it involved a black hole nearly six times the mass of our sun devouring a neutron star with a mass half again larger than our sun and that the merger took place between 650 million and 1.5 billion light-years away

GW200105 wasn't detected as definitively, but scientists suspect it was a merger between a black hole about nine times the mass of the sun and a neutron star about twice as massive as the sun about 550 million and 1.3 billion light-years away.

Scientists aren't sure yet whether these mixed mergers create a visible light signal (as neutron star pairs merging seem to do) or not (as in the case of binary black hole mergers).

The astronomers were not able to match these new gravitational wave detections with lightwave observations, but this does not necessarily mean there was no corresponding flash. For the less precise detection, scientists could only narrow down the location of the source to about 17% of the sky; for the more precise detection, scientists were still confronting an area the equivalent of 2,900 full moons. Moreover, any light would have been extremely low at such vast distances from the collisions when it reached Earth anyway.

However, the scientists suspect that at least for these particular mergers, there was no light signal to see.

"These were not events where the black holes munched on the neutron stars like the Cookie Monster and flung bits and pieces," Patrick Brady, a physicist at the University of Wisconsin-Milwaukee and current spokesperson of the LIGO Scientific Collaboration, said in a statement. "This is what would produce light, and we do not believe this has happened in these cases.” (Disorderly food is also called tidal disturbance.)

Both events mark the first time that scientists witnessed a fusion and were convinced that it was a mixed couple. However, for two previous detections, the same scenario is possible, although neither can be confirmed by astronomers. One of these events, detected in August 2019, depicts a large black hole consisting of either the largest known neutron star or the smallest known black hole. Another event detected four months ago could be merging a mixed pair, but could simply represent the noise in the detectors.

Based on the two observations of January 2020, scientists are now predicting that a fusion between a black hole and a neutron star occurs once a month within a billion light-years of Earth.

Scientists have two theories to explain how such amalgamations happen. One is that each member of a binary star goes independently supernova, exploding and forming two dense remains that end up merging. The other theory suggests that disparate stars are subjected to supernova explosions and then establish a binary relationship.

The two new collision observations are not sufficient to determine what is happening, but the scientists hope that in the long term, the detection of gravitational waves will solve the problem.

"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," Maya Fishbach, a postdoc at Northwestern University in Illinois and a co-author of the study, said in a university statement. "With future data on gravity waves, we will have the statistics to answer these questions and ultimately learn how the most extreme objects in our universe are made.”

The twin detectors LIGO, Virgo and KAGRA are all in preparation for the fourth round of observations of the partnership, which should begin next summer. Scientists say that work could see the partnership detecting one gravitational wave signal every day, opening scientists with immensely more information about what is taking place across the cosmos, as in these dramatic mergers.

"Every collision is not merely the gathering of two massive and dense objects. It's really like Pac-Man, with a black hole swallowing its companion neutron star whole," Susan Scott, a physicist at the Australian National University and co-author of the study, said in a university statement. "These collisions shook the universe to its center and we sensed the ripples they sent through the cosmos.”

The findings are described in an article in the June 29 issue of The Astrophysical Journal Letters.