1,200-Year-Old Cosmic Blast Captured in Japanese Trees
http://news.nationalgeographic.com/n...space-science/A surplus of radioactive atoms in Japanese trees may point to an unrecorded astronomical event that showered Earth with cosmic rays about 1,200 years ago.
This link also has a historical reference to a quote by a chronicler 776 AD about "fiery and fearful signs in the heavens":
Vast cosmic event leaves record in ancient trees
So it's 20 times more radiation than normal... I'm wondering how large a supernova could produce this. And no observations of the actual event? I'm thinking that maybe the possible supernova was visible only from Southern hemisphere so that would explain the lack of historical records of a new star? Or would the Earth act in this case as a radiation shield for Northern hemisphere?