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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.

 

These emanate from the Universe's most extreme environments and violent processes.

 

The Crab Nebula is composed mainly of the remnant of a supernova, which was seen on Earth to rip itself apart in the year 1054.

 

At the heart of the brilliantly coloured gas cloud 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, said Roger Blandford, director of the Kavli Institute for Particle Astrophysics and Cosmology, US.

 

"The origin of these high-energy gamma rays has to be some other source," he told BBC News.

 

"It takes about six years for light to cross the nebula, so it must be a very compact region in comparison to the size of the nebula that's producing these outbursts on the time scales of hours."

 

Since its launch nearly three years ago, Fermi has spotted three such outbursts, with the first two reported earlier this year at the American Astronomical Society meeting.

 

These events are unleashing gamma rays with energies of more than 100 million electron-volts – that is, each packet of light, or photon, carries tens of millions of times more energy than the light we can see.

 

But the Crab's recent outburst is more than five times more intense than any yet observed.

 

'Big puzzle'

What has perplexed astronomers is that these variations in gamma rays are not matched by changes in the emission of other light "colours". Follow-up studies using the Chandra X-ray telescope, for example, showed no variations in the X-ray intensity.

 

Kavli Institute researcher Rolf Buehler outlined the details of the Crab's flashes to the meeting on Thursday.

 

"If you look in optical light, the Crab is very steady; in radio emission, it's very steady; in very, very high-energy gamma rays it's very steady. Only in this part between do we see it varying," he told BBC News.

 

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