A Twinkling Fireball

HUNTSVILLE, ALABAMA--Astronomers have sized up what appears to be a giant fireball, billions of light-years away, and clocked it expanding at near the speed of light. The findings, which will be announced here tomorrow at the Fourth Huntsville Gamma Ray Burst Symposium, fit well with theory about the violent event--called a gamma-ray burst (GRB)--that could be triggered by the merging of two burnt-out stars.

The measurements, by Dale Frail and several colleagues at the National Radio Astronomy Observatory in Socorro, New Mexico, and other institutions gauged both the size and expansion speed by monitoring the twinkling of radio emission from the fireball after the GRB. The results implied a fireball moving outward at nearly the speed of light and reaching about a tenth of a light-year across after roughly 2 weeks--in general agreement with the fireball theory. "This measurement is the first confirmation that we are on the right track," says Tsvi Piran of Hebrew University in Israel.

The burst was first seen on 8 May by the orbiting Burst and Transient Source Experiment (BATSE), which collects high-energy gamma-rays that would otherwise be blocked by Earth's atmosphere. But BATSE can't precisely determine the direction in space from which the bursts arrive. That was left to the Italian-Dutch BeppoSAX satellite, whose instruments found an x-ray afterglow from a specific region of the sky, setting off a chase among optical and radio astronomers to see if they, too, could detect a gradually decaying afterglow.

A few days after the burst, Frail and colleagues succeeded in tracking the radio emissions. But the radio signature didn't show the gradual decay. "One day [the emission] was barely detectable; the next day it was a whopping bright source," says Frail. Then the group became aware of work by Jeremy Goodman of Princeton University pointing out that turbulence in the gases of the Milky Way would scatter the radio light--just as Earth's atmosphere scatters light from stars, making them twinkle. Objects that appear larger in the sky, like planets, smear out the effect of the turbulence and don't twinkle. So when the radio source stopped twinkling, Frail's team could measure its apparent size in the sky and, knowing its distance, calculate its actual size.

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