They're the Mary-Kate and Ashley Olsen of astronomy circles. The black holes that literally made waves in space-time when they collided 1.3 billion years ago may well be twins born from the same star, according to a new paper.
"It's the cosmic equivalent of a pregnant woman carrying twins inside her belly," says Harvard-Smithsonian Centre for Astrophysics astrophysicist Avi Loeb.
And, like the Olsens, the famous pair is non-identical: one black hole weighs seven times the mass of the Sun more than the other.
In a paper published on Arxiv and accepted for publication in The Astrophysical Journal Letters, Loeb analysed a gamma-ray burst picked up by NASA's Fermi Space Telescope a fraction of a second after the gravitational wave rolled through Earth in September 2015.
Astronomers think black holes form when the core of a massive star, many times the size of the Sun, collapses in its death throes.
But if the original star is particularly big and spinning wildly, its core might not neatly compress in a sphere; rather, it could stretch into a dumbbell shape.
And as it spins, the ends of the dumbbell might pull apart completely, forming two independent, but neighbouring, black holes, Loeb says.
Within minutes, the black holes merged, erupting in a cataclysmic eruption that rippled space-time, and was picked up by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the US.
But the story doesn't end there. After merging, Loeb says, the now-single black hole voraciously fed on leftover scraps of matter from the original star's dying explosion, consuming up to a Sun's worth of material each second, and blasted jets of matter outwards to create a gamma-ray burst.
It's this gamma-ray burst, he writes, that's the telltale signature of the death of a massive star capable of forming two black holes.
While the Fermi telescope spotted the burst, and pinpointed its location to be in the same patch of sky as the black holes, another gamma-ray-detecting satellite, the European INTEGRAL satellite, did not detect anything.
"Even if the Fermi detection is a false alarm, future LIGO events should be monitored for accompanying light irrespective of whether they originate from black holes," Loeb says.
"Nature can always surprise us."