Shedding light on a gamma-ray mystery

In a matter of seconds, gamma-ray bursts generate some of the most energetic fireworks in the universe, then vanish without a trace. In the 3 decades since the first burst was observed, several satellites have detected about 2,000 of these high-energy flashes, which occur uniformly throughout the sky. Yet astronomers still don't know whether the bursts originate within our home galaxy or far beyond.

 This enduring puzzle may soon be solved. For the first time, researchers believe they have spied the visible-light afterglow of a gamma-ray burst. Within a week, that ember had faded, but astronomers subsequently identified a faint galaxy in the same position. "It may turn out that a breakthrough is in the making," says theorist Bohdan Paczynski of Princeton University.

 The saga began Feb. 28, when the Dutch-Italian satellite BeppoSAX detected a bright gamma-ray burst. The satellite, launched last April, has X-ray telescopes that immediately home in on the lower-energy tail of a burst to provide an accurate position for astronomers attempting to observe the flash at other wavelengths.

 Eight hours later, while directed toward the patch of sky that contained the burst, a group of higher-resolution telescopes aboard BeppoSAX glimpsed a rapidly fading X-ray source thought to represent the burst as it cooled. Enrico Costa of the Space Astrophysics Institute in Frascati, Italy, and his colleagues reported the finding in a March 1 circular of the International Astronomical Union (IAU).

 That announcement sparked a flurry of activity. On March 1, astronomers using two telescopes in the Canary Islands, Spain, discovered a visible-light object that coincided with the fading X-ray source. Intriguingly, observations 7 days later revealed that the object had faded from view, Jan van Paradijs of the University of Amsterdam and the University of Alabama, Huntsville and his collaborators report in a March 12 IAU circular.

 Using the New Technology Telescope in La Silla, Chile, the researchers found a galaxy at the same position, they report in a March 14 circular.

 Mark R. Metzger of the California Institute of Technology in Pasadena and his colleagues report also finding a galaxy in that place during observations with the Keck II Telescope atop Hawaii's Mauna Kea.

 Researchers must still consider the possibility that the burst actually occurred in our own galaxy and just happened to coincide with a faint, distant galaxy. Fortunately, says Abraham Loeb of Harvard University, "we don't need to speculate for long on this matter. BeppoSAX is expected to find such a [burst] event every month." If subsequent events show associations with other galaxies, he notes, it may clinch the case for the extragalactic origin of bursts.