Anno Domini 774

A mysterious source of radiation was captured in tree rings sometime from 774 to 775.  Scientists say it wasn't solar flares or supernovae; it's some mystery what it might have been.

I'd just like to note the entry from the Anglo-Saxon Chronicle for that year.

This year the Northumbrians banished their king, Alred, from York at Easter-tide; and chose Ethelred, the son of Mull, for their lord, who reigned four winters. This year also appeared in the heavens a red crucifix, after sunset; the Mercians and the men of Kent fought at Otford; and wonderful serpents were seen in the land of the South-Saxons.

Now, the Anglo-Saxon Chronicle is a pretty sober document.  Most of the entries were made by monks, recording the chief events of the year.  However, once in a while one does get a surprising claim -- for example, see the entry for the year A.D. 793, the year the Vikings first appeared in England and plundered the holy island of Lindisfarne.

Still, in spite of the occasional entry that contemporary readers are inclined to reject, it's generally a reliable source for information.  If they say a red crucifix appeared in the heavens that year, I'd be inclined to consider that as a possible physical description of whatever it is that caused the strangeness in the tree rings.