May 11, 2011

Crab Nebula Astonishes With Gamma-Ray Outbursts

A gamma-ray burst released by the Crab Nebula on April 12 has shocked astronomers, reports BBC News.

The Crab Nebula was once thought to be the steadiest high-energy source in the sky.

"For 40 years, most astronomers regarded the Crab as a standard candle," Colleen Wilson-Hodge, an astrophysicist at NASA's Marshall Space Flight Center in Huntsville, Ala., said in a previous statement posted in January.

"Now, for the first time, we're clearly seeing how much our candle flickers," she said.

The Crab Nebula is the wreckage of an exploded star whose light reached Earth in 1054.  It is one of the most studied objects in the sky. 

NASA said at the heart of an expanding gas cloud lies what is left of the original star's cone, a superdense neutron star that spins 30 times a second.

"The Crab Nebula is a cornerstone of high-energy astrophysics, and this study shows us that our foundation is slightly askew," team member Mike Cherry at Louisiana State University said in the same statement.

The team analyzed observations made by the Gamma-ray Burst Monitor (GBM) aboard NASA's Fermi Gamma-ray Space Telescope.

Since Fermi's launch three years ago, it has spotted three big outburst, with the first two reported earlier this year at the American Astronomical Society meeting.

These events are unleashing gamma rays with energies of over 100 million electron volts.

However, the Crab Nebula's recent outburst is over five times more intense than any yet observed.

Kavli Institute researcher Rolf Buehler outlined the details of the Crab's flashes during a 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.

"That's why people hadn't found this before; there was not an instrument like Fermi sensitive enough to capture it."

However, Buehler said understanding the flare may take some time.

"To have something that puts almost all of its energy into gamma rays is an unusual thing," he told BBC's Jason Palmer. "We're looking at a big puzzle and are probably going to need a couple of years to understand it."

Fermi project scientists Julie McEnery said that the find was a testament to the power of the Fermi telescope to elucidate new physics in the cosmos.

"It's just so extraordinary that so many telescopes over so many years have been looking at the Crab and it's been constant all that time, and suddenly we discover that it's not," she told BBC News.

"With Fermi, we have the opportunity to catch it when it's in this extraordinarily flaring state - it really brings home the advantage of having an instrument that looks at the whole sky all the time, because you catch the unexpected."


Image Caption: The Crab Nebula seen in infrared by the Hubble Space Telescope. Credit: NASA/ESA


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