Enlarge / A representation of a neutron star merger.
Two papers, two different conclusions.
The one thing everyone can agree on is that something strange happened in the year 774, and that whatever it was sent a burst of high-energy particles into the Earth's atmosphere. Exactly what that event was, however, has remained the subject of contention. And it's back in the news today, with a new study pointing the finger at a rare event called a short gamma-ray burst.
The reason for the part that people agree on is an unusually large spike in the amount of radioactive carbon found in tree rings that have been dated back to 774. That apparently is correlated with the timing of a surge in a specific isotope of beryllium, detected in ice cores of Antarctica. Both of these isotopes are the product of collisions that take place in our atmosphere, produced by energetic particles striking some of the gasses normally resident there.
The obvious candidate, and the one that got everyone excited, was a nearby supernova. Unfortunately, supernovae that are close enough tend to be rather obvious. With a single exception (a mention of a "red crucifix" in the skies over Britain), nobody seems to have noticed anything unusual. Even more problematic, most supernovae leave a remnant, comprised of a hot, expanding cloud of material, with either a neutron star or black hole at its center. We've now done whole-sky surveys in the X-ray part of the spectrum, and we've not seen a remnant at the right age and distance.
That's where things stood late last year, when a group of scientists caught a basic logical flaw in the initial research. The original description of the isotope anomaly ruled out the Sun because it doesn't have big enough eruptions of energetic particles. But the calculations assumed the eruption would have been spread evenly in all directions; the Sun's eruptions are actually directional, so the total energy involved in the event is much smaller.
New calculations brought the total energy down to about 20 times the size of the largest event in recorded human history. Although we've never seen the Sun do anything on that scale, we have seen eruptions like that on other Sun-like stars in our galaxy.
That would seem to provide a plausible explanation for the surge in these two isotopes. But the recalculation of the solar flare strength was only published in December; by that point, the paper that came out yesterday was probably through peer review. And it takes an entirely different tack, focusing on more extreme cosmic events.
Short-period gamma-ray bursts are generally thought to result from the merger of two compact objects, like white dwarfs or neutron stars. This process can occur without a big boom, and thus won't produce a debris field. But it can produce a short and intense burst of extremely high energy photons. To give an example from the paper, "For example, the merger of two magnetized neutron stars can produce a spinning black hole launching a relativistic jet as observed in short GRB [gamma-ray bursts]."
The authors of the new paper run the numbers, and gamma ray bursts have been observed with the right sort of energies to create the isotopes we see, provided that the event occurred within the Milky Way or one of its satellite galaxies. And, thanks to the observations of their frequency in the Universe in general, we can calculate how often that they occur in our own galaxy. That comes up with a figure of about one every 3.7 million years, although the error range is about as large as the figure itself.
Doing the calculations a different way, however, gives us a very different figure. Based on the number of binary systems of compact objects, we can estimate that mergers take place about once every 5,000 years. That would be right in the range of the error from the earlier calculation. But there's a small problem: in most cases, the beams generated during the merger wouldn't be pointing at Earth. So, the real number is about one event like this every 40,000 years.
Despite the enormous uncertainties, the authors like their model. A lot. They like it so much, in fact, that they conclude that the isotope excursion "is the first evidence for a short GRB in our Galaxy." Of course, they hadn't seen the more recent paper that pinned the blame on the Sun when they wrote that. The BBC got in touch with one of the authors of that paper, and he was very skeptical, telling the broadcaster, "A solar proton event and a short gamma-ray burst are both possible explanations, but based on the rates that we know about in the Universe, the gamma-ray burst explanation is about 10,000 times less likely to be true in that time period."
The problem, of course, is that either option is going to be exceedingly rare, and nobody with a knowledge of modern astronomy was around to get the details when it did. And, given how reliant we are on all the orbiting electronics that would be in the path of a similar event, I doubt anybody wants to be around to witness a repeat.
Monthly Notices of the Royal Astronomical Society, 2013. DOI: 10.1093/mnras/sts378 (About DOIs).
