Could the Earth have been hit by a gamma ray burst?

An artist's impression of the merger of two neutron stars. Short duration gamma-ray bursts are thought to be caused by the merger of some combination of white dwarfs, neutron stars or black holes.

According to a new study published yesterday, the Earth may have been hit by one of the most powerful blasts of radiation in the known universe — a gamma ray burst — sometime in the late 8th century.

Gamma ray bursts are intense flashes of radiation that are thought to be produced when a massive star dies and collapses to form a neutron star or black hole, and they are considered the brightest events in the known universe. One of the potential "extinction-level events" that could end all life on the Earth is having a gamma ray burst from a nearby star (maybe 30 light years away) blast our solar system. That would destroy a significant amount of the ozone layer on the side of the planet facing the direction of the burst, exposing every living thing on the surface of the Earth to lethal levels of radiation from the Sun.

The idea that the Earth was hit by a gamma ray burst in the past came from the discovery of high levels of the isotopes Carbon-14 and Beryllium-10 in tree rings formed around 775 CE (Common Era). Since these two isotopes are created when nitrogen is hit with radiation from space, the researchers began searching historical accounts for a source of this event.

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Solar flares were ruled out because any flares powerful enough to produce the levels of radiation necessary would have been accompanied by coronal mass ejections that would caused intense auroras around the north and south poles. No accounts of increased aurora activity were recorded, though. One possible supernova seen was described in the Anglo-Saxon Chronicle as "a red crucifix, after sunset," which occurred in the year 774 CE (although several other articles I've read about this state it was in 776 CE).

It is possible that this 'red crucifix' was the cause, however no remnant has been seen from this apparent supernova. This led to the conclusion that it must have been an event that would have left very little visual evidence afterward.

It is possible, according to the researchers, that two massive stellar remnants — black holes, neutron stars or white dwarf stars — may have collided and merged, throwing off a quick, intense pulse of gamma rays, but very little in the visible part of the light spectrum. This would account for both the presence of the isotopes and either the lack of a historical account (if the 'red crucifix' is a 'red herring'), or the brief appearance of the initial flash with no remnant detected afterwards.

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The study suggests that the explosion that caused the burst probably happened somewhere between 3,000 and 12,000 light years from here. At that distance, the radiation wouldn't have been intense enough to strip away the ozone layer, but it would still have produced the isotopes from nitrogen in our atmosphere.

"If the gamma ray burst had been much closer to the Earth it would have caused significant harm to the biosphere," said Dr. Ralph Neuhӓuser, an astronomer from the University of Jena in Germany, who was one of the authors of this study. "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 3000 years, the maximum age of trees alive today, only one such event appears to have taken place."

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