What Hit Earth 1,200 Years Ago?

Japanese scientists studying tree rings data found something strange: 1,200 years ago an extremely intense burst of high-energy radiation of unknown origin hit planet Earth.

The radiation burst, which seems to have hit between ad 774 and ad 775, was detected by looking at the amounts of the radioactive isotope carbon-14 in tree rings that formed during the ad 775 growing season in the Northern Hemisphere. The increase in 14C levels is so clear that the scientists, led by Fusa Miyake, a cosmic-ray physicist from Nagoya University in Japan, conclude that the atmospheric level of 14C must have jumped by 1.2% over the course of no longer than a year, about 20 times more than the normal rate of variation.

But what happened, exactly?

The only known events that can produce a 14C spike are floods of gamma-rays from supernova explosions or proton storms from giant solar flares. But neither seems likely, Miyake says, because each should have been large enough to have had other effects that would have been observed at the time.

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