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Tuning in a mysterious radio source.

Astronomers have discovered a fluctuating source of radio waves that may lie only a few light-years from our galaxy's center. The object, though still unnamed and uncategorized, may reveal new details about the Milky Way's core, which in visible light is hopelessly obscured by the dust between it and Earth.

Using the Very Large Array (VLA) radio telescope near Socorro, N.M., researchers first spotted the enigmatic emissions in late December. W. Miller Goss, Jun-Hui Zhao and their colleagues were making a routine survey of the well-known radio source Sagittarius [A sup.*], located at the precise center of the Milky Way, when they detected a point source of radio waves located about 5 light-years off-center. Two weeks earlier, the survey had shown no evidence of radio waves from this spot, indicating the celestial oddity was compact enough to "switch on" rapidly.

By late January, the object's radio emissions had intensified sharply, matching the output of Sagittarius [A.sup.*], the researchers reported in a March 16 circular of the International Astronomical Union. After a fairly steady and dramatic decline over the next several months, the emissions intensified slightly in early May, Goss told SCIENCE NEWS.

He and his collaborators have examined the source at three different radio wavelengths, using both the VLA and the partially completed Very Long Baseline Array. They now conclude that the emissions do not come from an ordinary star, but rather from the acceleration of high-speed electrons moving in a strong magnetic field near the galactic center.

The object's identity remains unknown, although the team has come up with two leading candidates: a supernova explosion or an X-ray binary star system. Over time, each of these would display a telltale emission pattern. Radio waves from a supernova should decay fairly steadily, explains Goss, while those from an X-ray binary should fluctaute, possibly in periodic cycles reflecting the time it takes for one member of the binary system to orbit the other.

A continued intensification of radio emissions in the coming months would favor the X-ray binary model, says Goss, who notes that binary systems may concentrate in high-density regions such as the galactic core, where the gravitational capture of one star by another is likely. X-ray binary systems feature a compact object -- usually a neutron star -- that pulls in mass from a larger, orbiting companion. As charged material, such as electrons, falls from the companion star onto its compact partner, the material can emit bursts of radio waves as well as X-rays, Goss says. Analyses of recent observations made with ROSAT, the British-German X-ray satellite, should reveal whether the new find is indeed an X-ray binary, he adds.

In any case, other astronomers have confirmed that it lies close to the galactic center. In March, says Goss, a team working at the Australia Telescope National Facility at Culgoora detected a molecular cloud, made of hydrogen and other gases, that absorbs certain radio emissions from this object. The cloud's absorption pattern matches that associated with other radio sources known to lie near the Milky Way's center. The researchers still can't tell, however, whether the radio source actually sits within the cloud or is a separate entity located somewhere behind it.
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Title Annotation:near the Milky Way's core
Publication:Science News
Date:May 25, 1991
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