Mysterious radiation burst recorded in tree rings

Spike in carbon-14 levels indicates a massive cosmic event — but supernovae and solar flares ruled out.
Just over 1,200 years ago, the planet was hit by an extremely intense burst of high-energy radiation of unknown cause, scientists studying tree-ring data have found.
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.


http://www.nature.com/news/mysterious-radiation-burst-recorded-in-tree-rings-1.10768