Astronomers believe they've worked out the source of a mysterious burst of gamma rays that irradiated the Earth in the 8th century.
In 2012, scientist Fusa Miyake announced the detection of high levels of the isotope carbon-14 and beryllium-10 in tree rings formed in 775 AD, suggesting that a burst of radiation struck the Earth in the year 774 or 775.
Carbon-14 and beryllium-10 form when radiation from space collides with nitrogen atoms, which then decay to these heavier forms of carbon and beryllium. But it was clear at the time the burst was detected that it couldn't have been caused by a nearby supernova, as there are no contemporary accounts to support this and no remnant has been found.
Professor Miyake also considered whether a solar flare could have been responsible - but these simply aren't powerful enough to cause the observed excess of carbon-14. Nor are there any historical records of the bright aurorae that would have been expected.
Some researchers pointed to an entry in the Anglo-Saxon Chronicle that describes a ‘red crucifix’ seen after sunset, and suggested this might be a supernova. But this dates from 776 - too late to account for the carbon-14 data - and still doesn't explain why no remnant has been detected.
But there is another explanation, which fits both the carbon-14 measurements and the absence of any recorded events in the sky. Two compact stellar remnants - black holes, neutron stars or white dwarfs - could have collided and merged together in a process that would also release gamma rays.
In these mergers, the burst of gamma rays is intense but short, typically lasting less than two seconds. These events are seen in other galaxies many times each year but, unlike long duration bursts, without any corresponding visible light.
If this is the explanation for the eighth century radiation burst, then the merging stars couldn't be any closer than about 3,000 light years, or it would have led to the extinction of some terrestrial life.
Based on the carbon-14 measurements, Hambaryan and Neuhӓuser believe the gamma ray burst originated in a system between 3,000 and 12,000 light years from the sun. If they're right, then this would explain why no records exist of a supernova or auroral display.
Other work suggests that some visible light is emitted during short gamma-ray bursts that could be seen in a relatively nearby event. This might only be seen for a few days and be easily missed - but, say the authors, it might be worthwhile for historians to look again through contemporary texts.
Astronomers could also look for the merged object, a 1,200 year old black hole or neutron star between 3,000 and 12,000 light years from the sun but without the characteristic gas and dust of a supernova remnant.
"If the gamma ray burst had been much closer to the Earth it would have caused significant harm to the biosphere. 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," says Ralph Neuhӓuser of the University of Jena in Germany.
"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."