Why was Earth bombarded with high-energy particles in the year 774?

A representation of a neutron star merger.

NASA

The one thing everyone can agree on is that something strange happened in the year 774, and that whatever it was sent a burst of high-energy particles into the Earth's atmosphere. Exactly what that event was, however, has remained the subject of contention. And, it's back in the news today, with a new study pointing the finger at a rare event called a short gamma-ray burst.

The reason for the part that people agree on is an unusually large spike in the amount of radioactive carbon found in tree rings that have been dated back to 774. That apparently is correlated with the timing of a surge in a specific isotope of beryllium, detected in ice cores of Antarctica. Both of these isotopes are the product of collisions that take place in our atmosphere, produced by energetic particles striking some of the gasses normally resident there.

The obvious candidate, and one that got everyone excited, is a nearby supernova. Unfortunately, supernovae that are close enough tend to be rather obvious. With a single exception (a mention of a "red crucifix" in the skies over Britain), nobody seems to have noticed anything unusual. Even more problematic, most supernovae leave a remnant, comprised of a hot, expanding cloud of material, with either a neutron star or black hole at its center. We've now done whole-sky surveys in the X-ray part of the spectrum, and we've not seen a remnant at the right age and distance.