An analysis of Japanese tree indicated a sharp increase in their carbon-14 to carbon-12 ratio i the past. By using tree rings as a guide, it was determined that this happened in the years 774-775 A.D. This suggests that Earth was likely hit by a gamma-ray burst, or a violent cosmic blast of energy, about 1,200 years ago.
This is nothing to panic over. If it did indeed happen, it was quite a long time ago and is not likely to happen again for hundreds of thousands of years. And if it does, we know that the one 1,200 years ago did not cause any sort of mass extinction.
This mysterious wave of cosmic radiation smashed into the Earth in the eighth century may have come from two black holes colliding, says a study published in a British journal on Monday.
Japanese astrophysicist Fusa Miyake discovered the surge in carbon-14, an isotope that derives from high-energy radiation, in the rings of ancient cedar trees. So researchers set out to determine the nature of the radiation and what caused it.
There was no evidence that a supernova, or exploding star, occurred at that time, so that was ruled out as a cause. Researchers also ruled out a tantrum by the Sun, which can throw out cosmic waves or gouts of energy known as solar flares. Instead of the usual suspects, Valeri Hambaryan and Ralph Neuhaeuser of Germany came up with a new explanation, which they detail in Monthly Notices, a journal of Britain's Royal Astronomical Society.
Hambaryan and Neuhasuser, both of the University of Jena's Astrophysics Institute, suggest instead that two black holes collided and then merged, releasing an intense but very brief burst of gamma rays. Another possible cause could have been a collision of neutron stars, or "white dwarfs", which are compact stars near the end of their lives. These types of mergers are often spotted in other galaxies and do not typically generate visible light.
The researchers say that the event in 774 or 775 A.D. could have only taken place at least 3,000 light years away because otherwise the planet would have fried. The event occurring at this distance would explain why there is no evidence of an extinction event in Earth's biodiversity of the time, and why there is no record of an ultra-brilliant event in the sky.
The paper also suggests that astronomers should scour the skies because invisible remnants of the event could very well exist today.
Estimating the risk from a possible future collision of this kind could also be very important. Neuhaeuser explained:
"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. 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."