Craft spies new class of gamma-ray sources

Gamma rays are to visible light what slam dancing is to the fox trot. These high-energy emissions reveal some of the most violent choreography in the universe—the birth of black holes, the explosive collapse of ordinary stars, and the collision of charged particles spewed by fierce stellar winds. Such theatrics elude detection at the more quiescent wavelengths of visible light.

Map of unidentified gamma-ray sources greater than 100 million electron-volts. The brightest lie along the plane of the Milky Way. Fainter sources fit a curve associated with the Gould belt of stars. Gehrels et al.

It now appears that some of the drama revealed by gamma rays rages in our own neighborhood. Roughly half the 120 unidentified sources of high-energy gamma-ray emissions in the Milky Way—those at midgalactic latitudes—may come from a swath of massive stars that lies only a few hundred light-years from the solar system. This structure, known as the Gould belt, passes through the galactic plane at a 20º angle.

Those 60 or so gamma-ray sources may comprise a new class of objects, distinct from the 60 other mystery sources sprinkled along the plane of our galaxy, assert Neil Gehrels and his colleagues at NASA’s Goddard Space Flight Center in Greenbelt, Md. They describe their findings in the March 23 Nature.

Astronomers had previously noted that the distribution of the midlatitude gamma-ray sources, recorded by the EGRET telescope aboard NASA’s Compton Gamma Ray Observatory (GRO), coincides with the Gould belt. A new analysis strengthens that association and reveals that those sources have lower energy and are intrinsically fainter than those that lie along the galactic plane.

“Together, the findings provide compelling evidence that the belt is an important birthplace of gamma-ray sources,” says Isabelle A. Grenier of the University of Paris and the Centre d’Etudes de Saclay in Gif-sur-Yvette, France, in the same issue of Nature.

The midlatitude emissions have lower luminosity because they are generated by a different process than the emissions produced along the plane, Gehrels suggests. The gamma rays in the plane may come from strong magnetic fields possessed by the dense remains of massive stars thousands of light-years distant. However, midlatitude gamma rays may arise from collisions of charged particles in the powerful winds of active, rapidly rotating stars in the Gould belt.

The apparent proximity of the midlatitude sources will offer a unique view of how particles are accelerated and how gamma rays are produced, Gehrels says.

He cautions, however, that because astronomers can’t measure the distance to the gamma-ray sources, the association with the Gould belt remains uncertain. Current gamma-ray telescopes can’t pinpoint the location of sources to better than 1 square degree, twice the size of the sun as it appears on the sky, Grenier says. Scheduled for launch in 2005, the Gamma-Ray Large Area Space Telescope will have at least 10 times that positional accuracy.

In the meantime, GRO may have had its last hurrah. One of its three gyroscopes failed last December. If another dies, engineers may not be able to control the craft’s reentry through Earth’s atmosphere. The space agency will soon decide whether to bring down GRO in a few months.

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