Cosmic radiation is constantly bombarding our planet, fragmenting atoms in the upper atmosphere as it does. Every now and again, however, vast cosmic events can have a more dramatic effect on our world.
Occasionally considered as having potentially catastrophic consequences, gamma ray bursts are amongst the most powerful single events in the known universe – so powerful that we can detect them from the other side of the Universe. Now it looks as if Earth was struck by one about 1,200 years ago.
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Last year, a researcher named Fusa Miyake discovered high levels of carbon-14 in the rings of ancient cedar trees in Japan. Carbon-14 is a rare isotope of carbon, and the tree rings enabled it to be accurately dated to the year 775 CE. This also coincided with high levels of beryllium-10 found in Antarctica and dated to the same time. Some clever deductive reasoning by Valeri Hambaryan and Ralph Neuhäuser suggests that a gamma ray burst might have been to blame.
Cosmic rays striking our atmosphere cause nitrogen atoms to fragment and decay, producing both carbon-14 and beryllium-10, but the levels of these two isotopes were over ten times as high as normal galactic cosmic rays could explain. In order to explain the amount of carbon-14 observed, it was found that gamma rays would need a total energy of 700 quadrillion Joules - equivalent to 167.3 megatons of TNT!
Of course, there are other events which could be responsible, but none of them seem to fit. An extra powerful solar flare could supply enough energy, but the energy needed was about 20 times that that would be expected from the solar cycle at the time. Solar flares also blast Earth with charged particles which would create dramatic aurorae over Earth's North and South Poles. But no such aurorae were recorded in any historic texts.
A nearby supernova could also be to blame. Supernovae are reliably recorded in historic texts, often referred to as "guest stars". However there's no mention of any supernova at the time. While it's possible for the visible light from a supernova to be blocked by interstellar dust, that can't be the case here; if a supernova was behind the carbon-14 in 775 CE, it would have had to be close. Close enough that there isn't enough interstellar dust to block out all of the light. What's more, astronomers would have found the remnant of a supernova that close and recent by now.
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A third discounted possibility is that of a magnetar flare - a sudden burst of x-rays and gamma rays from a nearby neutron star with a powerful magnetic field. Magnetars occasionally suffer from starquakes. Violent ruptures in the surface of the neutron star, which disturb the magnetic field and cause powerful gamma ray emissions. Such magnetar flares have been detected here on Earth before (there have been three since the 1970s). Once again, the problem with this idea is that there are no neutron stars close enough to Earth to deliver such a powerful blast of gamma rays.
Hambaryan and Neuhäuser found that the only thing which could properly explain the carbon-14 was a short gamma ray burst - and after reading all of the other possibilities they considered, they make a fairly convincing argument. Lasting for just a couple of seconds, short bursts are believed to be typically caused by two neutron stars colliding together. Because neutron stars are surprisingly small, they collide rapidly, obliterating each other and usually forming a black hole in the process.
If a short gamma ray burst was responsible for the 775 CE event, then it must have been around 3,000 – 12,000 light-years away. Any closer, and it would probably have caused the extinction of some life on Earth. If the hypothesis turns out to be true, then the culprit is still at large. Astronomers could attempt to identify any suspects by looking for a black hole, aged about 1,200 years, within 120,000 light-years from Earth.
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While events like these are exceptionally rare, Dr Neunhäuser notes that if such a blast of gamma rays were to hit Earth today, "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."
While we only know of one such event having occurred in recorded history, we have no idea how frequent they may truly be. However, other unexplained events have been recorded elsewhere, in Antarctic ice for example. For now though, we can rest assured that gamma ray bursts are rare in our galaxy, so it's unlikely that we might be affected by one anytime soon.
Image: An artists impression of a dust enshrouded gamma ray burst – ESO/L. Calçada