It's been widely reported that the gravitational waves detected by LIGO, the Laser Interferometer Gravitational-Wave Observatory, were generated when two enormous black holes spiralled around each other and merged.

However, there's another interesting aspect of the discovery—something mysterious that scientists aren't able to explain. It seems that the collision may have been followed by a bright burst of gamma rays.

Belly of the Beast
Credit: R. Hurt/Caltech-JPL

No one expected such a bright gamma-ray burst to accompany a black-hole merger. In fact, isolated black holes can't ignite them. Instead, the black holes clear out a region of empty space as they spiral around each other.

Despite this, NASA's Fermi gamma-ray space telescope detected such an eruption of gamma rays 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.

“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.”

Avi Loeb of Harvard University thinks he has an explanation: The black holes may formed and merged within the belly of a star a few hundred times larger than our Sun, which would have resulted in such a gamma-ray burst.  “It’s sort of like a pregnant woman with twins in her belly,” he says. Once the black holes merged, the star's core would collapse and trigger intense beams of gamma rays.

Normally, that would form a single black hole, but if the star were rotating fast, centrifugal force would stretch the collapsing core, shaping it into a dumbbell. Eventually, the dumbbell would snap into two cores, each of which would continue to collapse into its own black hole.

New Thoughts Needed

It's important to note that this is just a hypothesis - and it's too soon to know whether it's correct. Right now, though, scientists don't have an alternative possibility.

“The only way to explain the Fermi signal is to surround the black holes with a lot of dense material, and the obvious way to do that, as in Loeb’s idea, is to put them inside a star,” says Dan Maoz of Tel Aviv University in Israel. “Maybe there are other ideas, but we need to think them up.”

There is one other possibility: the gamma-ray signal could be non-existent. Researchers analyzing data from the European gamma-ray spacecraft, INTEGRAL, couldn’t corroborate Fermi’s finding, and concluded that the gamma ray signal is not real. However, this doesn't mean that, in the future, there could be real signals that do coincide with a LIGO event, Loeb says.

“We shouldn’t have a prejudice that black hole binaries are always silent in terms of their electromagnetic signature.”

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