Did an eighth-century gamma-ray burst irradiate Earth?

The detection of high levels of the isotope carbon-14 and beryllium-10 in tree rings formed in A.D. 775 suggest that a burst of radiation struck Earth in the year 774 or 775.

By Royal Astronomical Society, United Kingdom — Published: January 21, 2013

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. Theory suggests that they are short lived as there is little dust and gas to fuel an "afterglow." // Credit: Part of an image from NASA/Dana Berry

A nearby short-duration gamma-ray burst may be the cause of an intense blast of high-energy radiation that hit Earth in the eighth century, according to new research led by astronomers Valeri Hambaryan and Ralph Neuhauser, both from the Astrophysics Institute of the University of Jena in Germany.

In 2012, Fusa Miyake from Nagoya University in Japan announced the detection of high levels of the isotope carbon-14 and beryllium-10 in tree rings formed in A.D. 775, suggesting that a burst of radiation struck Earth in the year 774 or 775. Carbon-14 and beryllium-10 form when radiation from space collides with nitrogen atoms, which then decay to these heavier forms of carbon and beryllium. The earlier research ruled out the nearby explosion of a massive star — a supernova — as nothing was recorded in observations at the time and no remnant has been found.

Miyake also considered whether a solar flare could have been responsible, but these are not powerful enough to cause the observed excess of carbon-14. Large flares are likely to be accompanied by ejections of material from the Sun's corona, leading to vivid displays of the northern and southern lights — aurorae — but again no historical records suggest these took place.

Following this announcement, researchers pointed to an entry in the Anglo-Saxon Chronicle that describes a “red crucifix” seen after sunset and suggested this might be a supernova. But this dates from 776, too late to account for the carbon-14 data and still does not explain why no remnant has been detected.

Hambaryan and Neuhauser have another explanation, consistent with both the carbon-14 measurements and the absence of any recorded events in the sky. They suggest that two compact stellar remnants — i.e., black holes, neutron stars, or white dwarfs — collided and merged together. When this happens, some energy is released in the form of gamma rays, the most energetic part of the electromagnetic spectrum that includes visible light.

In these mergers, the burst of gamma rays is intense but short, typically lasting less than two seconds. These events are seen in other galaxies many times each year but, in contrast to long-duration bursts, without any corresponding visible light. If this is the explanation for the 774/775-radiation burst, then the merging stars could not be closer than about 3,000 light-years, or the event would have led to the extinction of some terrestrial life. Based on the carbon-14 measurements, Hambaryan and Neuhauser believe the gamma-ray burst originated in a system between 3,000 and 12,000 light-years from the Sun.

If they are right, then this would explain why no records exist of a supernova or auroral display. Other work suggests that some visible light is emitted during short gamma-ray bursts that could be seen in a relatively nearby event. This might only be seen for a few days and be easily missed, but, nonetheless, it may be worthwhile for historians to look again through contemporary texts.

Astronomers also could look for the merged object, a 1,200-year-old black hole or neutron star 3,000 to 12,000 light-years from the Sun but without the characteristic gas and dust of a supernova remnant.

"If the gamma-ray burst had been much closer to the Earth, it would have caused significant harm to the biosphere,” said Neuhauser. “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."

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JAMES L STEPHENS from WASHINGTON said:

There could be another reason no visible sky source was noticed, although it doesn't explain why no Auroras were seen. The source could have been close to the sun in terms of viewing angle.

Submitted: 1/25/2013 4:53:12 PM (CST)

KENNETH E EBELING from IOWA said:

I have a question. A previous comment mentioned mass extinctions. If the distance range is 3000 to 12,000 LY, is it possible that a gamma ray burst may have been responsible for the extinction of the great Ice Age Mammals of the northern Hemmisphere in the past? The sudden loss of these mammals has been attributed to human hunting and recently an idea that an impact by a comet or asteroid on the existing glaciers, that would not have left an impact crater as the glaceir melted. This could be another possiblility??????

Submitted: 1/25/2013 11:03:26 AM (CST)

T MCGRATH from ALASKA said:

It should be noted that there has never been an observed collision between neutron stars or black holes, and it is only theorized that such a collision produces short gamma-ray bursts. While we have detected short gamma-ray bursts, their origin is still unknown.

Also, since white dwarf stars have a maximum mass of 1.44 solar masses, they are insufficient in mass to produce a gamma-ray burst even if they did collide.

The primary reason why Hambaryan and Neuhauser suggest the gamma-ray burst was caused by colliding neutron stars or black holes is because there were no recorded observations. Considering that this occurred in 774 AD or 775 AD, is it really that surprising that there were no recorded observations? The time period places it during the "Dark Ages", about 300 years after the fall of Rome, when Charlemagne ruled Europe. Astronomy actually lost ground during this period in Europe.

While the article does suggest that a large Coronal Mass Ejection could have caused the gamma-ray burst, which would have also resulted in auroras being visible in lower latitudes, it dismisses it as a possibility because it was not recorded. How many auroras have been recorded prior to 1859?

I think it is far more likely that a large CME may be responsible for the gamma-ray burst, and the event was observed but not recorded. Or perhaps it was recorded, and the records have been lost. In either case, there is more than one possible explanation for the gamma-ray burst.

Submitted: 1/25/2013 7:49:43 AM (CST)

BILL SIMPSON from LOUISIANA said:

Yet another hazard to consider. It makes you wonder how many are close enough to potentially do damage, and if they could be discovered in advance. Since they are small and potentially dim, it would seem to be very difficult. Three thousand light years has to contain a lot of stars. How many, is the question?
Of course, you have to ask if any of the known mass extinctions were cause by this kind of interaction event. Since we know that mass extinctions are quite rare, the probability of one originating from this type of event would seem to be quite low. If not, you would think that more mass extinctions would have happened in the past. Yet thankfully, the fossil record shows them to be rather rare events. (Now watch one hit next week.)

Submitted: 1/22/2013 11:10:48 PM (CST)

	Mercury	Mercury	Venus	Venus
RISE	—		—
SET	—		—
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Did an eighth-century gamma-ray burst irradiate Earth?

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