A giant burst of cosmic energy hit the Earth about 1,200 years ago, two astronomers theorize in a new report — and while some are skeptical of their findings, elemental changes in the Earth itself lend credence to their findings.

The Earth may have been hit by a gamma-ray burst, a violent cosmic blast of energy, about 1,200 years ago, according to a report in Oxford Journals.

Though the report was greeted with skepticim by some, the astronomers involved, Valeri Hambaryan and Ralph Neuhauser, make a compelling case. Here’s how it goes:

An analysis of Japanese trees indicates a sharp increase in their carbon-14 to carbon-12 ratio in the past. Using the tree rings as a guide, this occurred in the years 774-775 AD. What does this mean?

Most of the carbon around us is the carbon-12 flavor: Each atom has six protons and six neutrons in its nucleus. There’s a different kind of carbon, called carbon-14, which has, instead, eight neutrons and six protons. Carbon-14 is radioactive and decays into nitrogen over time. The fact that there’s any carbon-14 on Earth means it must be made continuously to resupply the atoms that go away, and this is done by cosmic rays hitting nitrogen in our atmosphere.

This just balances the amount that decays — think of it like a sink where you run the tap at the right amount to balance the water that drains away. The water level in the sink remains steady.

So we expect a certain amount of carbon-14 all around us. The Japanese trees have a sharp spike in them, about 10 times as much as usual. This also corresponds in time with a rise of carbon-14 seen in American and European trees, though the exact date is harder to pin down. It’s as if that sink you have suddenly has 10 times as much water in it. That water had to come from somewhere.

Not only that, but at the same time, something increased the amount of beryllium-10 (another radioactive element) in Antarctic ice by about 10 percent. That’s enough to be significant as well. What can make these big changes in elements all over the globe at the same time?

The best way to affect the whole Earth at the same time is to have something occur in space. It would take an extremely energetic cosmic catastrophe to do it, something that could dump a lot of energy into Earth’s atmosphere. There are a few different things that could do this: a giant solar flare, a nearby star exploding or a gamma-ray burst.

A solar flare is unlikely; the energy needed to create the carbon-14 detected would be 20 times larger than any solar flare ever seen. That’s possible, but the odds are pretty slim.

A nearby exploding star, or supernova, is almost certainly not the culprit. To generate the energy needed to create the carbon and beryllium seen, it would have to have been less than 1,000 light years away. That would’ve made it so bright it would’ve been visible in daylight. Also, no 1,200-year-old supernova remnant has been detected, and it would be incredibly obvious if it existed.

The astronomers studying this even looked at a magnetar flare, but it would’ve been only 100 light years away or so to do the deed, and anything that close would’ve been seen a long time ago.

That leaves a gamma-ray burst.

Born from a black hole

A gamma-ray burst (GRB) happens when a black hole is born. There are several ways this can occur; the most common is for an extremely massive star to explode at the end of its life. The core collapses to form a black hole, and the forces involved send out two colossal beams of energy into space. If they’re pointed our way, we see a flash of high-energy gamma rays.

In this case, this kind of GRB was ruled out due to the ratio of the carbon and beryllium detected — had it been this flavor of GRB, the ratio of carbon to beryllium created would’ve been much lower than what was seen.

There is another kind of GRB, though: the merger of two neutron stars. Imagine two massive stars orbiting each other. One ends its life as a supernova. The core collapses, but doesn’t have the oomph needed to make a black hole. Instead, it forms a neutron star, an incredibly dense ball of neutrons a few kilometers across but with the mass of the sun.

Then, sometime later, the second star explodes and also forms a neutron star. The two compact and incredibly dense objects orbit each other, and over time, the orbit decays. The two neutron stars get closer, until, eventually, they get so close they merge. The gravity of either star is a billion times that of Earth, so when they merge, it’s a fiercely violent event. There’s a huge explosion, and again you get those twin beams of energy blasting out.

Are we in danger?

If the beams miss us, well, no harm no foul. But if they happen to be aimed at Earth, we see a very short burst of gamma rays. Mind you, we see these events all the time, but they happen in distant galaxies, billions of light years away. We need sophisticated and sensitive telescopes to see them at all.

But if one happened in our own galaxy, at a distance of about 3,000 to 13,000 light years, then it all fits. The energy of the gamma rays hitting us would have been the equivalent of the detonation of about 200 one-megaton nuclear bombs, a huge amount, but spread out over one-half the Earth.

Also, the energy would’ve been sufficient to create the carbon-14 and beryllium-10 seen in the Japanese trees and Antarctic ice, and in the right ratio.

This kind of burst lasts for literally two seconds at most, so it’s entirely possible no one would’ve seen it. And it doesn’t necessarily leave behind anything we could see now, like the expanding gas in a supernova. All that’s left is a black hole, dark and quiet.

This all does hang together, and it does seem like a nearby GRB from a pair of merging neutron stars could’ve been behind the blast.