Black holes blamed for eighth-century radiation burst in tree rings


A wave of cosmic radiation that smashed into Earth in the eighth century may have come from two black holes that collided, according to a study published Monday that seeks to clear up a mystery raised by a Japanese astrophysicist.

Clues for the strange event were unearthed last year by Fusa Miyake of Nagoya University, who discovered a surge in carbon-14, an isotope that derives from high-energy radiation, in the rings of ancient cedar trees.

Dating of the trees showed that the burst struck the Earth in either 774 or 775.

Finding out the nature of the radiation and what caused it proved difficult. Scientists lined up the usual suspects only to let them go. There was no evidence that a supernova occurred at that time, they found. Also ruled out was a tantrum by our sun, which can throw out sizzling cosmic rays and solar flares.

Writing in Monthly Notices, a journal of Britain’s Royal Astronomical Society, German-based scientists Valeri Hambaryan and Ralph Neuhaeuser have come up with a new explanation.

They suggest that two black holes collided and merged, releasing an intense but extremely brief burst of gamma rays.

A collision of neutron stars or white dwarfs — tiny, compact stars near the end of their lives — may also have been the cause, according to Hambaryan and Neuhaeuser of the University of Jena’s Astrophysics Institute.

Mergers of this kind are often spotted in galaxies other than our own Milky Way.

The event in 774 or 775 could only have taken place at least 3,000 light-years from here, otherwise the planet would have fried, their paper says.

Estimating the risk from this kind of event could be vital.

“If the gamma ray burst had been much closer to the Earth it would have caused significant harm to the biosphere,” Neuhaeuser said. “But even thousands of light-years away, a similar event today could cause havoc with the sensitive electronic systems that advanced societies have come to depend on.”