Tree ring evidence suggests that a rare gamma-ray burst hit Earth in AD 774-775.
According to recent evidence, Earth’s so-called dark ages may have been briefly interrupted by the most powerful known explosion in the universe– a gamma ray burst.
Unusually high levels of the isotope carbon-14 discovered in the rings of ancient Japanese cedar trees align with a surge in beryllium-10 in Antarctic ice at the same time, suggesting that the Earth was subjected to a rare and powerful flash of cosmic radiation in AD 774 or 775.
Researchers believe the explosion was caused by a collision of two neutron stars, or two other compact stellar bodies such as black holes or white dwarfs. As the two massive cores collided and fused into one, the merger may have released a two-second blast of gamma rays far into the surrounding cosmos.
If the same levels of gamma radiation were to bombard the Earth today, it could knock out electronic systems all over the world. In the heart of the Middle Ages, little harm was done because of the great distance between our planet and the explosion.
“If the gamma-ray burst had been much closer to the Earth it would have caused significant harm to the biosphere,” said Dr, Ralph Neuhauser of the University of Jena in Germany. “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”.
According to the team’s scientists, the explosion was caused when two stellar remnants collided somewhere within the Milky Way. Neuhauser and his colleagues have ruled out the possibility of a nearby supernova, since evidence from tree rings did not suggest one, and historical records would have likely mentioned such a noticeable event. Records from the Anglo-Saxon Chronicle describe a “red crucifix” in 776, but that event appears to be one year too late. Regardless, a gamma burst emits no visible light, and would have likely gone unnoticed by humans.
High carbon-14 concentrations in tree rings provide a reliable indicator of cosmic-ray activity, according to the researchers who published the study. The radioactive isotope is formed when atmospheric nitrogen interacts with cosmic-ray neutrons. The amount of carbon-14 discovered in the AD 774-775 layers of two separate Japanese cedars was about twenty times larger than what is normally expected to result from solar modulation.
Gamma-ray bursts are exceedingly rare, with only a few occurring every million years. That is a good thing. A burst similar to that of AD 774-774, if it were within 200 light-years or so of Earth, would strip away our ozone layer on contact, likely leading to the extinction of most species on our planet, including our own. At the safe distance of 3,000 to 12,000 light years, however, our atmosphere and magnetic field would have done their usual 24-hour job of shielding us from harmful cosmic radiation– even though the unusually massive power of a gamma-ray burst managed to push some cosmic rays through to Earth’s surface.
Moving forward, researchers are hoping to learn more about the frequency and potential threat posed by gamma-ray bursts. “The challenge now is to establish how rare such carbon-14 spikes are, i.e., how often such radiation bursts hit the Earth,” said Neuhauser. “In the last 3,000 years, the maximum age of trees alive today, only one such event appears to have taken place.”
The group’s findings were announced Monday in the Monthly Notices of Britain’s Royal Astronomical Society journal, and based on research conducted in 2012.