Its detection on cue confirmed the model, suggesting that OJ287 was a black hole binary.

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"The detection of a binary black hole supports the idea that black holes can grow to enormous masses in the centres of galaxies by merging with other black holes, " said Komossa.

BBC: Black holes on collision course

"This is the first time we see a binary black hole in action, the smoking gun for something that will become a major gravitational wave burst in the future, " says Hasinger.

BBC: Black holes on collision course

The black holes are separated by an estimated distance of less than a third of a light-year - cheek-to-cheek by black hole standards and significantly closer than the postulated binary system spotted by the Chandra X-ray Observatory in 2003.

BBC: Dancing black hole twins spotted

Ascertaining the binary nature of the system also allowed the astronomers to make their accurate measurement of the bigger black hole's mass.

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