A few weeks back I wrote about Carbon 14 and how it was used to determine how old organic material is. Carbon 14 is created in the atmosphere by radioactive activity from space and is absorbed by living creatures. So astronomical activity is used by archaeologists to date things on Earth. Well now things might be working the other way around.
Now another way that we can date things from the past is to use tree rings. This is known as dendrochronology (which comes from the Greek involving trees and time). Each year most trees create rings of new material and we can tell them apart and count back to find ages. (I'll mention that “most trees” means that things like palm trees and Joshua Trees don't work the same way.) And since each year creates different growth patterns we can actually go back for thousands of years using different trees that have been calibrated to one long “ruler” for time.
Now simple Carbon 14 dating assumes that basically the rate of Carbon 14 production in the atmosphere is basically constant. But that's not the case, so when a group of Japanese scientists looked at the Carbon 14 levels in tree rings from 774-775 (A.D. or C.E.) they found ten times more Carbon 14 than expected. Since Carbon 14 is produced by astronomical events this means that the tree ring scientists had found a marker for some big astronomical event.
So the question became what kind of event? Things that had to be considered were solar flares, supernovas, and gamma ray bursts. Well, solar flares were quickly pushed to the side, since to have produced the observed amounts of Carbon 14 the solar flare would have had to have been much larger (many times) than any solar flare ever observed. Making a solar flare unlikely.
Next the supernova possibility was rejected. This was due to a couple of pieces of evidence. First the 8th century was one where historical records were being kept. And no record of a supernova was made at the time. This would have been any mention of a new bright star, or other reference to something remarkable in the sky. The other piece of evidence come from astronomy itself. Supernovas leave behind recognizable debris such as nebulae. And given the limitations on the speed of light, we know that if it was a supernova we would have found a nebula that was about 1200 light-years away. No such nebula exists.
So the scientist had to try to find something that had enough energy to create all the observed Carbon 14 but not have been observed. And that left the possibility of a gamma ray burst. Gamma ray bursts are incredibly energetic events that happen when either two black holes or two neutron stars collide. When this happens two powerful beams of gamma rays are shot out. And gamma rays are not visible to the eye so no one on Earth would have noticed the event. (In addition the bust last for mere seconds.)
But now astronomers are out looking for the remnants of a neutron star collision that took place between 3,000 and 13,000 light years away. Now it isn't clear whether they will find anything. It is still possible that there was some other cause to the Carbon 14 spike. But that's the way science works. A possible explanation is made, then they go out looking for something to confirm or refute that explanation. And either way it works out, science has gotten better at understanding the universe.