An artist’s impression of black holes about to collide
Mark Myers, ARC Centre ofExcellence for Gravitational Wave Discovery (OzGrav)
The Laser Interferometer Gravitational-wave Observatory (LIGO) and its partner detector Virgo have made their biggest find yet. They spotted two huge black holes smashing together to form another with a mass 142 times that of the sun, the biggest black hole detected using gravitational waves.
We have direct evidence for black holes both smaller and larger than these – stellar-mass black holes that can be dozens of times the mass of the sun and form as stars die, and supermassive black holes that are at least a million times as massive as the sun and sit at the centres of galaxies. This is the first direct confirmation of an intermediate-mass black hole.
“At masses between 60 and 130 solar masses or so, it’s impossible for a star to turn into a black hole, it just blows itself apart,” says LIGO team member Nelson Christensen at the Observatory of Nice in France. “Astrophysicists theorised that we’re not going to find any black holes in this gap (between stellar-mass and supermassive black holes) and we found at least one but maybe two.”
LIGO consists of a pair of enormous L-shaped detectors in the US, and Virgo is another detector in Italy. When massive objects in space move, they create ripples in space-time called gravitational waves that stretch and squeeze everything they pass, and the three detectors use that stretching and squeezing to determine what caused the ripples.
On 21 May 2019, all three detectors found gravitational waves from a pair of black holes that were about 65 and 85 times the mass of the sun, respectively, spiralling towards one another and merging. The result of this colossal collision was a single black hole 142 times the mass of the sun, with 8 solar masses worth of energy radiating away in the form of gravitational waves.
That means that these two black holes were probably not formed from stars, but were instead second-generation ones, formed by yet more pairs of smaller black holes, Christensen says.
“There has been indirect evidence for intermediate mass black holes, but this is a real observation of an event that’s definitely above 100 solar masses,” he says. “It’s confirmation that intermediate-mass black holes exist.”
We might even have an idea of where this black hole is lurking. Shortly after LIGO and Virgo detected the merger, the Zwicky Transient Facility (ZTF) in California spotted a burst of light from a galaxy close to where the gravitational wave measurements suggest the collision happened.
The burst of light came from near the centre of the galaxy, where a dense disc of matter circles a supermassive black hole. Because this type of region is so crowded, we expect many objects, including black holes, to smash together there as they orbit the galaxy’s centre, says Michael Coughlin at the University of Minnesota, who is part of the ZTF team. Then, as the larger final black hole travels through the disc, it would crash through other matter and cause a flare.
“The association is a little suspect: the distances don’t quite match and the locations are just on the edge,” says Coughlin. “But this thing’s gonna come around again, so it should cause another flare – that would be a smoking gun.”
References: Physical Review Letters, DOI: 10.1103/PhysRevLett.125.101102; arxiv.org/abs/2006.14122
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