A study last year found unusually high levels of the isotope carbon-14 in ancient rings of Japanese cedar trees and a corresponding spike in beryllium-10 in Antarctic ice.
The readings were traced back to a point in AD 774 or 775, suggesting that during that period the Earth was hit by an intense burst of radiation, but researchers were initially unable to determine its cause.
Now a separate team of astronomers have suggested it could have been due to the collision of two compact stellar remnants such as black holes, neutron stars or white dwarfs.
The crashing together and subsequent merger of the two bodies could have released an intense burst of energy in the form of gamma rays lasting as little as two seconds, they said.
It could have caused significant damage to ecosystems at the time but would be much more noticeable if it hit the Earth today, with the potential to bring down electronic systems across the globe.