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 as a "standard candle" to calibrate their instruments because for most of the time it gives off a steady stream of gamma rays and light of other wavelengths.
Two orbiting telescopes have revealed that the nebula, which sits some 6500 light years from Earth, releases brief, bright flares of gamma-rays. These flares are most likely produced by electrons that have been whipped up to record-breaking speeds. Now 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.