Analysis of two ancient trees found a surge in carbon-14 -- a carbon isotope that derives from cosmic radiation -- which occurred just in AD 774 and AD 775, the team report in the journal Nature on Sunday.
Earth is battered by protons and other sub-atomic particles which are blasted across space by high-energy sources.
The particles collide with the stratosphere and react with nitrogen to create carbon-14, which is then absorbed into the biosphere.
A team led by Fusa Miyake of Nagoya University found that levels of carbon-14 in the two cedars were about 1.2 percent higher in 774 and 775 compared to other years.
This may not sound much, but in relation to background concentrations of carbon-14, the difference is huge.
But Miyake's team say that the cosmic whack of 774-775 cannot be attributed to the Schwabe cycle of the time -- and it is far bigger than any known flare from the Sun.
The other possibility is a supernova, or a star that explodes at the end of its life in a welter of gamma radiation.