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A shower of gamma-ray findings.

Gamma rays pack more energy than any other radiation in the universe. Born during such cataclysms as supernova explosions, galactic collisions, and interactions between charged particles and monster magnetic fields, gamma rays may shed light on violent events throughout the universe. Last week, researchers presented the latest findings on these emissions from space--the only environment in which they are directly detectable -- based on data gathered by NASA's Compton Gamma Ray Observatory (GRO).

Detectors aboard the Earth-orbiting observatory, launched last April, have logged a total of 210 gamma-ray bursts so far, nearly double the number reported in September (SN: 9/28/91, p.196). Scattered across the sky, the highly uniform distribution of emissions from these cosmic flashbulbs continues to confound scientists, says Gerald J. Fishman of NASA's Marshall Space Flight Center in Huntsville, Ala.

Prevailing theory held that the bursts emerge from explosive events on or near ultradense, rapidly rotating neutron stars, which cluster along the plane of our galaxy. But the emissions detected by GRO's Burst and Transient Source Experiment (BATSE) show no evidence of such clustering, casting doubt on the theory.

Fishman cites three alternative explanations for the bursts, which typically last only a few seconds before vanishing, apparently forever. Unknown sources along the outskirts of the solar system could produce the uniform distribution, although such nearby bursters seem unlikely, he notes. On the other hand, sources may reside in an as-yet-unseen halo surrounding the Milky Way. The new data indicate that such a halo would have to lie at least 150,000 light-years from the center of our galaxy--about 1.5 times more distant than speculated last fall, Fishman says. Only at that distance could a halo centered on the Milky Way produce an array of bursts that would appear uniform even from Earth's off-center location, about two-thirds of the way out from the galaxy's core. The burst might also emanate from extragalactic bodies scattered throughout the observable universe, he adds.

BATSE has also detected a gamma-ray pulsar--only the third known example of such an object. Named the Circinus pulsar for the galaxy in which it resides, this rhythmically flashing neutron star rotates 400 times per minute and has a magnetic field trillions of times stronger than Earth's, Fishman says. Like its two siblings, it also radiates radio waves and X-rays, but gammas make up most of its pulsed energy.

The other two gamma-ray pulsars emit their gamma flashes twice during each rotation; scientists have speculated that one flash comes from the north magnetic pole, the other from the south. Circinus, in contrast, seems to flash only once per rotation. However, F. Curtis Michel of Rice University in Houston suggests that it may actually emit two flashes per rotation, with one flash obscured by the pulsar's orientation in relation to Earth.

Micheal adds that Circinus' gamma-ray flashes last longer than its radio-wave pulses - an observation that appears to support the theory that gammas originate higher above the pulsar's poles, where the magnetic field is whether, than do radio waves.

Data gathered by another GRO instrument, the Energetic Gamma-Ray Experiment Telescope, show that three previously discovered quasars are spewing so many gamma rays that each quasar emits about 10 million to 100 million times the total gamma-ray output of the Milky Way, reports Carl E. Fichtel of the Goddard Space Flight Center in Greenbelt, Md. These cosmic powerhouses lie between 10 billion and 20 billion light-years from Earth in the constellations Eridanus and Hercules and in a region near the Crab nebula, Fichtel says.
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Author:Cowen, Ron
Publication:Science News
Date:Jan 25, 1992
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