A gamma-ray burst's enduring fireball.

It's the cosmic ember that keeps on glowing.

Last March, astronomers spied for the first time in visible light the smoldering remnant of a gamma-ray burst--a titanic explosion that unleashes its fireworks in a matter of seconds (SN: 5/17/97, p. 305). This afterglow has since faded substantially, but it remains visible in images taken Sept. 5 by the Hubble Space Telescope.

The duration of the afterglow and its slow decline provide further evidence that bursts originate far beyond the Milky Way rather than inside it, Hubble scientists assert. Astronomers have long debated the origin of gamma-ray bursts.

A nearby burst carries much less energy than a faraway burst of the same observed brightness. Therefore, if the fireball imaged by Hubble, called GRB 970228, had erupted in the Milky Way, it would have packed less of a wallop and faded drastically after a few days, says Mario Livio of the Space Telescope Science Institute (STScI) in Baltimore.

"We've followed this [burst] for 6 months, and it continues to decay at the same rate," says Livio. "This strongly argues that the burst is distant." He points out that the fading fireball lies at the edge of a fuzzy blob that remains as bright as when Hubble imaged it in March. This suggests that the blob is a galaxy, perhaps the one in which the burst exploded, rather than material temporarily heated by the fireball.

Livio and his colleagues, including Andrew S. Fruchter and Kailash C. Sahu of STScI and Elena Pian of the Italian National Research Council in Bologna, reported the findings last week at a meeting in Huntsville, Ala., on gamma-ray bursts.

Last May, researchers measured for the first time the distance to a gammaray burst, called GRB 970508, proving that it resided far beyond the Milky Way (SN: 5/31/97, p. 335). Although scientists have compelling data on only 2 of the roughly 3,000 bursts detected, "there's no way to defend the [Milky Way] origin at the moment," says Bohdan Paczynski of Princeton University. The source of the bursts remains "anybody's guess," he adds.

With instruments on three spacecraft--the repaired BeppoSAX satellite, the Rossi X-ray Timing Explorer, and the Compton Gamma Ray Observatory--searching for flashes, an answer may not be long in coming.