Researchers have observed gravitational waves coming from the final interaction between a black hole and a neutron star merging for the first time, detecting not just once but twice in a matter of days. These were the first confirmed detections of this kind and show something very peculiar: the black holes swallowed their neutron stars whole.

The first merger, GW200105, was detected on January 5, 2020, by one of Earth's three gravitational wave observatories. It appeared consistent with a black hole with a mass nine times bigger than our Sun and a neutron star weighing 1.9 solar masses. The second, GW200115, was detected just 10 days later by all three LIGO and Virgo detectors and saw a black hole about six times the mass of the Sun and a neutron star with 1.5 times that mass.

As reported in The Astrophysical Journal, these two detections originated in two galaxies at a distance of roughly 900 million light-years from Earth. The team states that no light emission appeared to have been detected from these events, suggesting that given the size of the black holes, the neutron stars were not ripped apart as the two objects merged.

"These collisions have shaken the Universe to its core and we've detected the ripples they have sent hurtling through the cosmos," co-author Professor Susan Scott from the Australian National University, said in a statement.

"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."

Black holes this size and neutron stars are both the end product of supernovae, the explosive final stage in the lives of the most massive stars. Both are extreme objects and we still don’t have a complete understanding of either.

Gravitational waves have helped with that. Detections of mergers between two black holes and two neutron stars have expanded our knowledge, but a collision between a black hole and a neutron star has been "the elusive missing piece of the family picture of compact object mergers," said Chase Kimball, a Northwestern graduate student who co-authored the study. 

With these detections, we can finally look at mixed mergers and gain even more insights. Just a year ago, the announcement of a possible mixed merger between a black hole and either an extremely light black hole or the most massive neutron star yet teased the possibility of what was to come

"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 Professor Vicky Kalogera, director of the Center for Interdisciplinary Exploration and Research in Astrophysics at Northwestern University, in a statement.

"Quantitatively matching the rate constraints and properties for all three population types will be a powerful way to answer the foundational questions of origins."

But for that, we’ll have to wait until next summer. The two LIGO gravitational wave detectors in the US, and the European detector, Virgo, in Italy, will be joined by a new one, KAGRA, in Japan. Mixed mergers within 1 billion light-years are predicted to occur at least once a month, so although not all of these will be detectable, an extra detector will up the game.

"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," Professor Patrick Brady, spokesperson for the LIGO Scientific Collaboration, said. "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."