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Gamma-ray Burst Hits Highest Energy Yet

(Image - the Large Area Telescope)
The Large Area Telescope detects gamma-rays by tracking the electrons and positrons they produce after striking layers of tungsten. This ability also makes the LAT an excellent tool for exploring high-energy cosmic rays. (Image: NASA/Goddard Space Flight Center Conceptual Image Lab.)

For the second time in as many years, a Large Area Telescope collaboration meeting was punctuated by a stellar firework. Last week's meeting, which ran from August 29 through September 4, was briefly interrupted on Wednesday when the LAT, the main instrument onboard the Fermi Gamma-ray Space Telescope, recorded a large gamma-ray burst.

Although the Gamma-ray Burst Monitor, the other instrument aboard FGST, sees gamma-ray bursts almost daily, the LAT detects far fewer because it views only about a sixth of the sky at any given time and detects only the bursts that emit the highest-energy gamma rays. Including last week's burst, the LAT total now stands at ten.

When the alert came in last week notifying collaboration members of the possibility of a newly detected burst, the researchers leapt to action, alerting astronomers around the world so that they too could turn their instruments toward it. At the same time, the LAT automatically stopped its regular scan of the sky to continue recording the burst.

"We knew this was likely to be an exciting one—it was immediately clear that it was a very big burst," said SLAC astrophysicist and LAT collaboration member Jim Chiang.

Using additional data collected that afternoon, researchers determined that the burst included the highest energy gamma-ray so far measured from a gamma-ray burst: 33 GeV.

Roger Blandford, director of the Kavli Institute for Particle Astrophysics and Astrophysics, said that three main scientific messages can be gathered from this type of burst. "First, when you see high-energy gamma rays, it means the source must be rushing toward us with high speed. Second, from these gamma rays we've come to believe that most bursts are associated with the birth of a black hole in a supernova explosion." And third, Blandford said, observations are showing that short- and long-duration bursts, which were previously considered to be different in some fundamental way, from our perspective are looking increasingly similar. But that last understanding is still a work in progress, he said.

"The LAT is a superb instrument that keeps on giving," said Blandford. "It's outperformed our highest expectations in almost all areas. Everyone who's associated should take pride."

—Kelen Tuttle
SLAC Today, September 11, 2009