A study of ancient cedar trees in Japan suggests the Earth was struck by a blast of radiation during the Middle Ages and researchers think they know the source.
Researchers writing in the journal Monthly Notices of the Royal Astronomical Society say a gamma ray burst, the most powerful explosion known in the universe, may be responsible for the high levels of a radioactive type of carbon known as carbon-14 in the rings of the Japanese trees.
There has also been a discovery in Antarctica of a spike in levels of a form of beryllium in ice cores studies there, researchers said.
Both isotopes are created by intense radiation hitting atoms in Earth's upper atmosphere, suggesting a blast of energy hit our planet from space.
Researchers suggest it was the result of two black holes or neutron stars merging in our galaxy, the BBC reported Monday.
The tree-ring and ice-core data points to this happening between A.D. 774 and A.D. 775, researchers said.
"We looked in the spectra of short gamma-ray bursts to estimate whether this would be consistent with the production rate of carbon-14 and beryllium-10 that we observed -- and [we found] that is fully consistent," Ralph Neuhauser, from the Institute of Astrophysics at the University of Jena in Germany, said.
Gamma ray bursts are hundreds of times brighter than supernovae and about a million trillion times brighter than the sun.
"Gamma-ray bursts are very, very explosive and energetic events, and so we considered from the energy what would be the distance given the energy observed," Neuhauser said.
"Our conclusion was it was 3,000 to 12,000 light-years away -- and this is within our galaxy."
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