Quick reactions from astronomers last week may have settled a long-standing debate in astrophysics: the origin of the mysterious flashes of energy called gamma-ray bursts. After the Italian-Dutch satellite Beppo-SAX detected one on 8 May, rapidly adjusted telescopes on the ground spotted a quickly dimming smear of light. It came not from a source in or near our own galaxy, as one side of the argument would have it, but from the farthest reaches of the observable universe.
Last March, Dutch astronomers had found a similar optical counterpart to a gamma-ray burst detected by Beppo-SAX. The faint smudge of rapidly fading light looked like a distant galaxy. However, a few weeks later, a group led by Patrizia Caraveo at the Istituto di Fisica Cosmica in Milan, Italy, claimed that Hubble Space Telescope observations showed that the counterpart was moving across the sky--behavior that would not be expected unless the source lay within a few hundred light-years from Earth.
The 8 May burst tips the balance back in favor of the cosmological explanation. After Beppo-SAX spotted the burst, in the northern constellation the Giraffe, the observers sent word to astronomers around the world. Within a few hours, Howard Bond of the Space Telescope Science Institute in Baltimore had aimed a telescope at the Kitt Peak National Observatory in Arizona at the site of the burst. There he found an unusual source--first brightening, then dimming.
Then, on 11 May, Mark Metzger and his colleagues at the California Institute of Technology pointed the 10-meter Keck II telescope on Mauna Kea, Hawaii, at the faint optical source. They found dark lines in its spectrum--the shadow of material between the source and Earth. The lines were displaced far from their normal position, toward the red end of the spectrum, by the expansion of the universe. This so-called red shift, measured at 0.835, implied that the material causing the shadow is several billion light-years away--and the burst source is, if anything, even farther.
Fans of the cosmological model are ecstatic. "I think this is a victory," says Bohdan Paczynski of Princeton University, who quit working on gamma-ray bursts several years ago for lack of progress in the field. Now he's toying with the idea of studying the afterglow of these fleeting sources: "This seems to be a very promising and exciting area."