Mystery flares betray hidden force within Crab nebula

The particle accelerator to beat them all (Image: NASA, ESA, J. Hester, A. Loll (ASU))

Forget the Large Hadron Collider: an interstellar dust cloud called the Crab nebula has been identified as the most powerful known particle accelerator in the universe but exactly how it boosts particles to record-breaking speeds is a mystery.

The finding also adds an extra complication for astronomers who use the Crab to calibrate their instruments.

Two orbiting telescopes have revealed that the nebula, which sits some 6500 light years from Earth, releases brief, bright flares of gamma-rays, a pair of papers released today report. These flares are most likely produced by electrons that have been whipped up to record-breaking speeds.

These are "the highest particle energies ever associated with a single source", says Elisa Bernardini of the DESY research centre in Germany in an accompanying paper.

The discovery of the flares comes as a surprise. The Crab nebula is used as a "standard candle" to calibrate astronomical instruments because for most of the time it gives off a steady stream of gamma rays and other wavelengths.

It is made up of the gas and dust left over from a stellar explosion observed by Chinese astronomers in AD 1054, and the constant brightness is thought to be due to the pulsar, or dead spinning star, at the nebula's centre, which sends electrons slamming into the nebula. Magnetic fields help confine this electron stream to the nebula, but also slow down the electrons, causing them to emit light.

two studies suggest this light is not as steady as previously thought. The Italian Space Agency's AGILE satellite and NASA's Fermi Gamma-ray Space Telescope have together detected three bright gamma-ray outbursts from the nebula. The most recent outburst, in September 2010, lasted for four days and brightened the nebula by a factor of 6.

The outbursts are not likely to scupper use of the Crab nebula as a calibration tool, but astronomers will in future have to make sure they are not doing so during a period of unusual activity. "We're going to have to be more careful about using the flux from the Crab," says Stefan Funk of Stanford University in California, who is a member of the Fermi team.

The team estimates the electrons responsible for the bursts of light must have had energies of 1000 teraelectronvolts or more. That's about 100 times the energies that the protons inside the Large Hadron Collider – the world's most powerful particle accelerator - will attain when it reaches full power.

Exactly how the Crab nebula accelerates particles to such high energies is unclear. The magnetic fields that help confine the electrons to the nebula also cause them to criss-cross a shock wave created when particles streaming from the pulsar collide with the nebula. Each crossing gives the electrons an energy boost, but that can't be the whole story because the same fields will also bend the electrons' paths, sapping their energy before they reach record-breaking speeds.

"The standard methods of accelerating particles will not work, because they cannot accelerate particles rapidly," Funk says. "We have to invoke a different acceleration mechanism." Although it is not yet clear what might fit the bill, he says electric fields generated by the pulsar itself may help accelerate the electrons.