The Crab Nebula is the result of a supernova observed by Chinese astronomers in 1064, making it one of the more recent remnants available to study. Its core contains a rapidly spinning pulsar, which helps power a shockwave that produces emissions that run from the infrared, across the visible, and into the gamma-ray portion of the spectrum. In general, its output has been so consistent that astronomers have actually used it to calibrate orbiting observatories. But over the last few years, several of these observatories have picked up sudden surges in the nebula's output that hint at electrons traveling with energies of a Peta-electron Volt.

These outbursts don't come along that often—there seem to have only been three of them since 2007—and they last for just a handful of days. But we've now got a set of telescopes capable of picking up these events, and two of them, Fermi and AGILE, each caught two of the three (one occurred before Fermi was in orbit; AGILE was pointing in a different direction during another).

The events ranged from four days to about two weeks, and involved an increase in the gamma-ray output that was over eight standard deviations above the background. Most events we're aware of that can create photons with the energies detected take a long time to dissipate, so the relatively brief and erratic nature of these events limits the number of phenomena that can describe them. Both the AGILE and the Fermi groups conclude there's only one process that is likely to do the trick: synchrotron radiation.

This comes from when charged particles have their paths bent by electric or magnetic fields, causing them to emit radiation that's proportional to their speeds. In order to emit radiation that's in the gamma-ray region, however, these things have to be moving very, very fast. That's where the authors' estimate of over a PeV comes from. By way of contrast, The LHC is currently operating in the low TeV range.

And that's a bit of a problem: "These are the highest energy particles that can be associated with a discrete astronomical source, and they pose challenges to particle acceleration theory." In short, although we know there are electric fields around, we don't know how they could generate a field strong enough to push electrons around at these energies. "The Crab Nebula is powered by the central neutron star which acts as a DC unipolar inductor and a source of an AC striped wind," the authors write. "What happens to the DC and AC current flows is controversial." They hope that continued observations will catch enough of these events to figure out precisely where in the nebula they come from, and what physical structures that corresponds to.

Science, 2010. DOI: 10.1126/science.1199705, 10.1126/science.1200083 (About DOIs).