Duck! Too late...

Phil Plait has a Slate article about a hit 1,200 years ago:

Well now, here’s something I never thought I’d hear: the Earth may have been hit by a gamma-ray burst, a violent cosmic blast of energy, about 1200 years ago!
First off, this is nothing to panic about. If it happened at all, it was a long time ago, and unlikely to happen again for hundreds of thousands of years. And if it did happen, it didn’t cause any mass extinctions or anything like that— if it had, we’d have known about it earlier!
But it’s very interesting, and while I remain skeptical, the astronomers involved 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 (very high energy particles from space) 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 ten 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 ten 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 ten 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 (that is, violent) 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 twenty times larger than any solar flare ever seen. That’s possible, so I wouldn’t necessarily rule it out, 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 a thousand light years away. That would’ve made it so bright it would’ve been visible in daylight! Also, no 1200-year-old supernova remnant has been detected, and it would be incredibly obvious if it existed (the Crab nebula is a thousand years old and more than six thousand light years way, for example, and is one of the brightest supernova remnants in the sky).
The astronomers studying this even looked at a magnetar flare, but it would’ve been only a hundred 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. And that’s amazing.
A gamma-ray burst (GRB, for short) 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, like deadly lighthouse beams, into space. If they’re pointed our way we see a flash of high-energy gamma rays. Hence the name.
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 neutron 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 ridiculously dense objects orbit each other, and over time (due to complicated relativistic effects) 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.
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 three thousand to thirteen thousand light years, then it all fits. The energy of the gamma rays hitting us would have been the equivalent of the detonation of about two hundred one-megaton nuclear bombs, a huge amount, but spread out over one-half the Earth (and less devastating because there would not have been the huge fireball and radioactivity of an actual bomb). 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. This surprises me! We expect to see one of these events in our galaxy about once per million years or so, so having one happen just twelve hundred years ago seems unlikely. And it would have to have been aimed right at us; the beams from the explosion are narrow, so if it were aimed the wrong way we would’ve escaped the consequences entirely. But this idea is at least as likely as a ginormous solar flare, given we’ve never seen a flare anywhere near powerful enough to create those radioactive isotopes.
So, wow. I wrote a book on scenarios like this, called Death from the Skies! and, in one chapter I described what it would be like to have a nearby GRB go off. I put one much closer to the Earth— a mere hundred light years away— so the effects were, um, not so good (like, setting the Earth-on-fire not so good). But the farther away they are, the dimmer and less dangerous they are, and several thousand light years is a decent buffer. In fact, I’d consider that close for one in our galaxy, and it hardly had any lasting effect on our planet! Don’t get me wrong: If one of those went off now at that distance, it would be bad. Our atmosphere would absorb all the radiation and we’d be safe enough from all that here on Earth’s surface, but we’d lose satellites, the interaction of the high-energy gamma rays would blow out power grids all over the planet, and our civilization would be in big trouble.
But remember, these events are very, very unlikely. So much so that I’m still skeptical a GRB was the culprit in this case! And no matter what it was, it must’ve been pretty rare. We watch the sky all the time, and if these kinds of events were common we’d have seen one by now.
Still, it’s a reminder that the Universe is a dangerous place, and we’re not entirely safe just sitting here on our planet. We need to keep watching the skies, keep learning more, and increasing our understanding of what’s out there. If we don’t understand those dangers, we can’t hope to prevent them, or at least prepare ourselves in case something like this happens again. Protecting our power grid and satellites is possible, though expensive, and if we hope to be able to keep our civilization going, we’ll need to understand all the potential dangers out there.

Rico says he'll be just as happy to have this happen when he's not around any more...