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Gamma rays from the Crab nebula.

Gamma rays from the Crab nebula

The Crab nebula, the optical remnant of a stellar explosion observed more than 900 years ago, is already known as one of the strongest sources of X-rays and radio waves. Now a team of astronomers has established that the Crab nebula is also a steady source of highly energetic gamma rays. "We have seen a very clear signal from the Crab nebula," says Trevor C. Weekes of the Harvard-Smithsonian Center for astrophysics in Cambridge, Mass. "It's a completely steady source of gamma rays."

The energy source powering the crab nebula is most likely a rotating neutron star, which has a highmagnetic field and can accelerate electrons to relativistic energies. These accelerated electrons interact with low-energy photons in the region surrounding the neutron star to generate gamma rays. The resulting gamma rays typically have energies exceeding those achievable in any particle accelerator or collider on Earth.

The steadliness of the signal establishes the Crab nebula as a standard against which to measure other gamma-ray emissions. A number of research teams have previously detected high-energy gamma rays from cosmic sources, but nearly all these signals proved sporadic and unpredictable. The existence of a single, well-understood source of cosmic gamma rays may help astronomers interpret other, more puzzling gamma-ray observations.

"The Crab is a weak gamma-ray source, but it's the strongest source yet detected," Weekes says. "It provides a standard signal."
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Author:Peterson, Ivars
Publication:Science News
Date:Apr 28, 1990
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