Call it a gut reaction. The revolutionary discovery of space-time ripples may have come from two black holes colliding while inside the belly of an enormous star, whose subsequent collapse launched powerful jets of gamma rays.
Scientists already knew that the gravitational waves detected by LIGO, the Laser Interferometer Gravitational-Wave Observatory, were generated when two black holes – each about 30 times as massive as the sun – spiralled around each other and merged.
But now it seems that collision may have been followed by a bright burst of gamma rays. NASA’s Fermi gamma-ray space telescope detected such an eruption just 0.4 seconds after LIGO’s gravitational waves arrived at Earth. It’s not clear whether the same event triggered both signals, but the Fermi team calculated that the probability of a coincidence was just 0.0022.
The problem is that no one expected such a bright gamma-ray burst to accompany a black-hole merger. Coalescing black holes orbit each other in a cosmic do-si-do, clearing out a region of empty space. According to models of gamma-ray bursts, isolated black holes can’t ignite them.
“Everything smells like a short gamma-ray burst in our signal,” says Valerie Connaughton of the Fermi team. “And that’s a real problem in a way – you don’t expect this signal from merging black holes.”
But when Avi Loeb of Harvard University saw the Fermi results, he realised he knew what might be happening. You could get a gamma-ray burst if the two black holes were enveloped inside a very massive star. “It’s sort of like a pregnant woman with twins in her belly,” he says. Once the black holes merged, the star would collapse and trigger intense beams of gamma rays.
For that to happen, the two black holes would have to have formed inside an extremely massive star a few hundred times heftier than the sun. As the star exhausted its nuclear fuel, its core began to collapse. Normally that would form a single black hole.
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