The year is 774 A.D. A blast of energy from outer space hits Earth. No one alive then seems to have been aware of it. But the event was recorded—in the trees. Last year researchers announced that tree-ring data from 774 and 775 show a sudden spike in radioactive carbon 14. The uptick points to a burst of charged particles or high-energy radiation that struck Earth. But what caused it?

One suggestion is that a supernova could be the culprit. And medieval texts do record the appearance of a “red crucifix” glowing in the night sky, although the timing is uncertain. 

Another astrophysical possibility is a short gamma-ray burst, researchers report in the Monthly Notices of the Royal Astronomical Society. [Valeri Hambaryan and Ralph Neuhäuser, A Galactic short gamma-ray burst as cause for the 14C peak in AD 774/5] Short gamma-ray bursts occur when super-dense objects—say, two neutron stars—merge, giving off a quick blast of radiation.

As long as it was a few thousand light-years away, a short gamma-ray burst in the galaxy might not have caused great harm. And they don’t glow much in visible light, so it’s possible that no one would notice. One problem with the theory? Short gamma-ray bursts appear to be extremely rare in our galaxy. On second thought, that’s probably a good thing.

—John Matson

[The above text is a transcript of this podcast.]