Crab Nebula's Gamma-Ray Flare Mystifies Astronomers

The Crab Nebula has shocked astronomers by emitting an unprecedented blast of gamma rays, the highest-energy light in the Universe. The cause of the 12 April gamma-ray flare, described at the Third Fermi Symposium in Rome, is a total mystery. It seems to have come from a small area of the famous nebula, which is the wreckage from an exploded star. The object has long been considered a steady source of light, but the Fermi telescope hints at greater activity.

The gamma-ray emission lasted for some six days, hitting levels 30 times higher than normal and varying at times from hour to hour. While the sky abounds with light across all parts of the spectrum, Nasa's Fermi space observatory is designed to measure only the most energetic light: gamma rays. At the heart of the brilliantly coloured gas cloud of the Crab Nebula we can see in visible light, there is a pulsar - a rapidly spinning neutron star that emits radio waves which sweep past the Earth 30 times per second. But so far none of the nebula's known components can explain the signal Fermi sees.

Mystery Flares Betray Force within Crab Nebula

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.