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Gamma-Ray Telescope to Open New Window on Cosmic Explosions

New instrument will look at gamma rays blasted out by the most powerful events in the universe

NASA on Saturday is set to launch the next generation of space-based gamma-ray detectors. If all goes as planned, GLAST—for Gamma-ray Large Area Space Telescope—will within months begin to send back detailed, real-time data on the most energetic explosions and flare-ups the cosmos has to offer.

You may think of gamma rays as tools for scorching tumors or maybe as the stuff of comic book and TV lore that belted mild-mannered scientist Bruce Banner and turned him into the Incredible Hulk. In fact, gamma rays are very high energy x-rays produced when powerful energetic forces strongly accelerate electrons and send them hurtling through space. 

GLAST will fill a blind spot in researchers' view of the heavens by scanning a wide swath of gamma rays, including a stretch of spectrum never before observed by ground- or space-based telescopes. This portion of the high-energy electromagnetic spectrum, from about 10 to 100 GeV (billion electron volts), "is almost entirely unexplored territory," says Steven Ritz, GLAST's project scientist and an astrophysicist at the NASA Goddard Space Flight Center in Greenbelt, Md.

The objects in the universe known to produce large amounts of gamma rays are few but impressive. "If you see a gamma-ray source, you know you've got something really cool," says astrophysicist Julie McEnery, GLAST's deputy project scientist. Blazars—found in the centers of some galaxies—and gamma-ray bursts are two identified wellsprings of these high-energy rays. Blazars periodically flare when the supermassive black holes in some active galaxies' cores fill with dust and gas, releasing massive amounts of energy. Gamma-ray bursts are enigmatic flashes of radiation that can last from a few milliseconds up to minutes. Scientists suspect these bursts occur during extreme celestial events like neutron star collisions, but the jury is still out pending further GLAST observations.

GLAST's large-area telescope (LAT) monitors 20 percent of the sky at a given time. The satellite completes one orbit per hour, and after every two orbits it has taken a full scan of space as visible from Earth. This allows the satellite to see more of the sky at a much faster rate than its predecessor, the Energetic Gamma-Ray Experiment Telescope (EGRET), which NASA launched in 1991 on board the Compton Gamma-Ray Observatory. "With EGRET, it was possible that the most extraordinary thing in the gamma-ray sky could be happening somewhere else," McEnery says.

EGRET completed the first gamma-ray survey of the cosmos, but was ultimately unable to resolve much of what it saw—over half of the 271 gamma-ray sources it observed still remain unidentified. It was also blind to the highest energy gamma rays. When they struck the instrument's energy detector, these superstrong gamma rays generated x-rays that fooled the device into rejecting them as background radiation.

GLAST uses a more sophisticated segmented "anticoincidence detector" that distinguishes particles by charge and location within the instrument. The gamma rays that enter the LAT must first pass through this detector which, like a bouncer, makes sure that only gamma rays get in the door.

These admitted rays then strike a thin tungsten sheet that cleaves them into an electron and a positron (a twin electron, but with a positive charge). Next, a series of silicon chip layers track the path of the particle pairs to pinpoint the source of the rays. The particles finally strike a so-called calorimeter, a device made of cesium iodide that measures the energy of the ray captured by the orbiting observatory.

After GLAST launches, mission controllers will spend a couple of weeks slowly turning on and fine-tuning its instruments and sensors, followed by a month or so to make sure they are properly interpreting the data from LAT, the main telescope. Researchers should be able to monitor gamma-ray bursts almost immediately after GLAST comes online by using its second instrument, called the GLAST burst monitor (GBM). Made up of flat disks of sodium iodide, the GBM looks for brief, less energetic pulses that are much easier to interpret because they come on fast and fade equally quickly. These less intense, though nonetheless powerful, pulses will include gamma rays from within our own galaxy as well as those emitted by our sun. GBM will also view the entire sky—except for the pocket blocked by Earth.

GLAST is slated to scan the heavens for gamma rays for five years, but researchers hope the satellite may peer across the universe for a full decade. With each pass of the sky, astronomers hope to gather more information about faraway, gamma-bright objects, watching them evolve as their gamma-ray emissions change over time.

"Though we will be surveying the whole sky," Ritz says, "it's like we will be zooming in more with each orbit."

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