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It is always good to check your work several times, you never know what you might spot. That’s a rule that works for data as well. Thanks to a new analysis, researchers have discovered 10 new black holes mergers in previously analyzed data from the LIGO-Virgo gravitational wave collaborations – and they are all peculiar and very interesting finds.
The black hole collisions were missed first time round as they were hiding outside the detection threshold of the collaboration's originally programmed analysis. By expanding the search, astrophysicists reanalyzed data collected during the O3a run of the two LIGO and the Virgo gravitational-wave observatories in 2019 that had already found 44 events and found 10 more.
One fascinating one was GW190521, which had two black holes around 66 and 85 solar masses. When certain massive stars explode in a supernova, they can turn into black holes but nuclear physics models suggest black holes created by stars are either less than about 50 solar masses or more than 150 and shouldn’t be in between.
These black holes in the upper mass gap are likely the results of previous mergers. The new data has found even more examples of these events, suggesting that they are likely more common than previously thought.
“When we find a black hole in this mass range, it tells us there’s more to the story of how the system formed since there is a good chance that an upper mass gap black hole is the product of a previous merger,” lead author Seth Olsen, a PhD candidate at Princeton University, said in a statement.
But there’s also a lower mass gap, covering the region with the heaviest neutron stars and the lightest black holes, with the data revealing new examples of these limiting collisions.
If these important findings weren’t enough already, the team also found a collision type that had never been seen before: a heavy black hole merged with a much smaller one, and their spins were not aligned. The spin of the heavy one went one way, and the smaller one went almost in the complete opposite direction.
“The heavier black hole's spin isn't exactly anti-aligned with the orbit,” Olsen explained, “but rather tilted somewhere between sideways and upside down, which tells us that this system may come from an interesting subpopulation of BBH mergers where the angles between BBH orbits and the black hole spins are all random."
More details on the new findings will be shared in a presentation on April 11 at the APS April Meeting 2022.
“With gravitational waves, we’re now starting to observe the wide variety of black holes that have merged over the last few billion years,” Olsen stated.
