A mystery wave of cosmic radiation that smashed into Earth in the eighth century may have come from two black holes that collided, a study published on Monday says.

Clues for the strange event were unearthed last year by Japanese astrophysicist Fusa Miyake, who discovered a surge in carbon-14 -- an isotope that derives from high-energy radiation -- in the rings of ancient cedar trees.

Dating of the trees showed that the burst struck the Earth in either 774 or 775 AD.

But what was the nature of the radiation, and what caused it?

Space scientists lined up the usual suspects only to let them go. There was no evidence that an exploding star, also called a supernova, occurred at that time, they found.

The Anglo-Saxon Chronicle, a record in Old English, makes a dramatic reference to the appearance of a "red crucifix" seen in the skies after sunset. But that happened in 776 AD, which was too late to tally with the event marked by the tree rings.

Also ruled out was a tantrum by the Sun, which can throw out sizzling cosmic rays or gouts of energy called solar flares.

Writing in Monthly Notices, a journal of Britain's Royal Astronomical Society, German-based scientists Valeri Hambaryan and Ralph Neuhaeuser have come up with a new explanation.

The pair suggest that two black holes collided and then merged, releasing an intense but extremely brief burst of gamma rays.

A collision of neutron stars or "white dwarf" stars (tiny, compact stars near the end of their lives) may also have been the cause, say Hambaryan and Neuhaeuser of the University of Jena's Astrophysics Institute.

Mergers of this kind are often spotted in galaxies other than our own Milky Way, and do not generate visible light.

The event in 774 or 775 AD could only have taken place at least 3,000 light years from here, otherwise the planet would have fried, says the paper.

If their theory is right, this would explain why there is no record of some ultra-brilliant event in the sky, or evidence of any extinction event in Earth's biodiversity at that time.

Astronomers should scour the skies because invisible remnants of the event could well exist today, the paper suggests.

And estimating the risk from a future collision of this kind could be vital.

"If the gamma ray burst had been much closer to the Earth it would have caused significant harm to the biosphere," explains Neuhaeuser.

"But even thousands of light years away, a similar event today could cause havoc with the sensitive electronic systems that advanced societies have come to depend on. The challenge now is to establish how rare such carbon-14 spikes are, i.e. how often such radiation bursts hit the Earth.

"In the last 3,000 years, the maximum age of trees alive today, only one such event appears to have taken place."