The international LIGO-Virgo-KAGRA collaboration has announced the observations of a whopping 35 new events that released gravitational waves, tiny vibrations in the fabric of space-time. These gravitational wave events are caused by cosmic collisions, with 32 of them likely involving two black holes smashing into each other. Two more events likely involved a rarer black hole and neutron star collision, and the final one has scientists uncertain as to exactly what type of compact object was involved.
This is the second half of the third observing run for the LIGO and Virgo gravitational wave observatories that were switched on and studying the universe between November 2019 and March 2020. The data release was uploaded on the paper repository ArXiv and has been submitted for publication together with three companion papers. It brings the total number of gravitational wave detections so far to 90.
These 35 new events cover the entirety of the mass range of gravitational waves that have been detected so far, meaning black holes of all shapes and sizes have been observed colliding; from neutron stars just slightly heavier than our Sun (but compressed into a sphere the size of a town) to black holes over 100 times heavier than our Sun.
“Only now are we starting to appreciate the wonderful diversity of black holes and neutron stars. Our latest results prove that they come in many sizes and combinations," Christopher Berry, a member of the LIGO Scientific Collaboration, said in a statement. “We have solved some long-standing mysteries but uncovered some new puzzles too. Using these observations, we are closer to unlocking the mysteries of how stars — the building blocks of our universe — evolve.”
Among the peculiar events, there’s one that involved a massive black hole (33 solar masses) with a very low-mass neutron star (about 1.17 solar masses). This is one of the lowest mass neutron stars ever observed, using either gravitational wave observatories or conventional (electromagnetic) telescopes.
The one uncertain merger event is believed to be between a black hole 24 times the mass of our Sun and a much lighter black hole around 2.8 solar masses. But while the researchers believe that a light black hole fits the profile better, a very heavy neutron star couldn’t be excluded. This is similar to a previous puzzling event recorded by the collaboration.
“Each new observing run brings new discoveries and surprises. The third observing run saw gravitational wave detection becoming an everyday thing, but I still think each detection is exciting!" Hannah Middleton, a postdoctoral researcher at OzGrav, University of Melbourne, said in a statement.
"Future observing runs are sure to find more unusual events – gravitational-wave astronomers have plenty to look forward to."
The two LIGO observatories (located in the US) and Virgo (located in Italy) are currently off and undergoing some upgrades in preparation for their fourth observing run. The trio will be joined by the KAGRA observatory in Japan, and all four will be online in late 2022.
“It turns out that the gravitational-wave universe is incredibly exciting,” said Maya Fishbach, a NASA Einstein Postdoctoral Fellow and member of the collaboration. “Our upgraded detectors will be able to catch quieter signals, including black holes and neutron stars that merged even farther away, with signals from billions of years ago. I can’t wait to discover what else is out there.”