8th-century tree rings hint at close-range space blast

A blast of radiation that hit Earth circa AD 770 may have been caused not by a solar flare but by the energetic debris from the collision of two nearby neutron stars.

Last year, Fusa Miyake at Nagoya University, Japan, and colleagues discovered that two Japanese cedar trees had unexpectedly high levels of carbon-14 in tree rings formed between 774 and 775. Normally levels of the isotope differ by just 0.05 per cent annually, but Miyake found a 1.2 per cent leap in those years that could only have been caused by extremely high-energy cosmic rays hitting the Earth.

Similar rises in carbon-14 have been found in American and European trees from the same era, while Antarctic ice cores from 775 also have increases in beryllium-10, another isotope caused by cosmic rays. There is nothing similar anywhere else in around 3000 years of tree ring records, leading Miyake to suggest a massive solar flare as the cause.

Not everyone is convinced, however, as such a bright blast of energy would surely have been noted at the time, and there is no historical record of such an energetic solar flare. "The aurora would have been seen up to tropical latitudes," says Valeri Hambaryan of the University of Jena, Germany.

Invisible crash

Some researchers have suggested that an unknown supernova might be responsible for a "red crucifix" in the sky, recorded in the Anglo-Saxon Chronicle, a history of Anglo-Saxon life compiled by anonymous scribes. If that were the case, the stellar remnants would still be visible today.

That is why Hambaryan and his colleague Ralph Neuhauser are now proposing a short gamma-ray burst (GRB) as the cause. Unlike a solar burst, it would have been invisible to the naked eye. GRBs are known to occur in distant galaxies and thought to be caused by the collision of two neutron stars, black holes or white dwarfs.

The pair suggest that the odd isotope levels in the trees and in the Antarctic ice are the first evidence for a burst much closer to home. They suggest looking for a neutron star between 3000 and 12,000 light years away left over from such a merger.

"I think their explanation is very interesting," says Miyake. However, he points out that as short gamma-ray bursts are thought to be rare, the chance of one happening in the relatively short 3000-year-period recorded by the Japanese trees is slim. GRBs could of course just be more common than we think. "They claim it is possible that more bursts exist than those observed to explain the inconsistency, however additional studies are necessary to confirm this claim," says Miyake.

Journal reference: Monthly Notices of the Royal Astronomical Society, DOI: 10.1093/mnras/sts378

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