A student asked this question about black holes during a discussion, and I didn’t have a good answer. Now there’s this:
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.
— via The Weather Channel (2013): Black Hole Collision May Have Irradiated Earth in 8th Century.
From the original article:
While long [Gamma Ray Bursts (GRBs)] are caused by the core collapse of a very massive star, short GRBs are explained by the merger of two compact objects … [such as] a neutron star with either a black hole becoming a more massive black hole, or with another neutron star becoming either a relatively massive stable neutron star or otherwise a black hole.
— Hambaryan and Neuhäuser (2013): A Galactic short gamma-ray burst as cause for the 14C peak in AD 774/5 in
More info via The Telegraph, and the original article discussing the spike in carbon-14 in tree rings is here.