Around 900 million years ago, two cosmic colosusses collided—and the encounter ended very badly for one of them.
In a tentative first, researchers think they’ve observed an elusive astronomical encounter: gravitational waves released by a black hole swallowing a neutron star, or the dense, collapsed remnants of a stellar explosion. If confirmed, the detection, made on August 14 by a scientists at the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo interferometer, will constitute the first concrete observation of such a merger in space, backing up decades of predictions.
So far, one thing is certain: the gravitational waves themselves. Prolonged, powerful ripples in the fabric of spacetime on the scale of those detected only occur when something truly cataclysmic, like a collision or collapse, happens in the cosmos.
“Something has occurred out there in the sky,” LIGO team member Daniel Holz of the University of Chicago told Emily Conover at Science News. “So far, it doesn’t obviously look like anything we’ve detected with high confidence before.”
Of course, puzzling out what the detection isn’t is a bit easier than pinpointing what it is. Still, the researchers are hopeful. LIGO, based in Washington state, and Virgo, which is in Italy, have picked up gravitational waves before, unveiling mergers between pairs of black holes, as well as a collision between two neutron stars. But the newest observation, called S190814bv, could well be the first definitive detection of a hybrid rendezvous.
Observations in April hinted at a neutron star-black hole merger, but the signal was weak, and several researchers cautioned that it may have been a false alarm. S190814bv, on the other hand, carries a lot more certainty. Already, the team behind the find is busily gathering follow-up data on the region of space in which the collision likely occurred.
One thing researchers are on the lookout for is any light left in the wake of the merger, which may have been emitted by the neutron star if it was shredded up by the black hole before getting gulped down. If that’s the case, it may give astronomers their first glimpse into the inner workings of a neutron star—which “would be fantastic [and] dream-like for a theorist,” LIGO team member Vicky Kalogera, a physicist at Northwestern University, told National Geographic’s Michael Greshko.
If there’s no afterglow, the neutron star may have been swallowed intact. But it’s also possible this hungry black hole didn’t gobble up a neutron star at all. Rather, it may have been an especially petite black hole, with a mass similar to that of a neutron star.
If that’s the case, this particular collision might be an act of cannibalism. But because a black hole this puny has never been found, S190814bv would still be a first—and could set a new lower limit for the mass of a black hole.
As LIGO team member Christopher Berry, a physicist at Northwestern University, told Greshko, “It’s a win-win situation.”