Star Death Spawns Gamma Ray Bursts

Astronomers think they have spotted the celestial powerhouses that generate the mysterious blasts of radiation known as gamma ray bursts. Three groups, who describe their results in this week's issue of Nature and in papers posted on the Internet, report evidence that the exploding stars called supernovae are responsible for the short-lived blasts, which have been one of astronomy's most vexing puzzles.

Although the first gamma ray bursts were detected 30 years ago, it was not until 1997 that astronomers managed to link a burst to a visible object. That was when a team led by Jan van Paradijs of the University of Amsterdam spotted a fading light source exactly at the location of a gamma ray burst (ScienceNOW, 17 March 1997). Thanks to a rapid alert system, other timely observations of gamma ray bursts soon followed. But even though astronomers could now see the embers of the celestial explosions, they did not know what powered them. In one theory, the burst of radiation was loosed when two superdense neutron stars collided; another attributed them to the explosion of stars tens of times more massive than the sun.

Evidence favoring the second idea has now come from a gamma ray burst spotted last March. Astronomers from the California Institute of Technology (Caltech) in Pasadena traced the fading afterglow for 1 month, then shifted their attention elsewhere. "We thought we knew everything about this burst," says team member Joshua Bloom. But in December they took another look to measure the galaxy in which they thought the burst had occurred, "and very quickly we realized that this galaxy had disappeared." This meant that the light they had originally seen could have been from a fading supernova, says Bloom.

To reconstruct the events, the team went back and pored over the optical images that had been taken for 30 days after the burst. They found that the initially blue afterglow gave way to a reddish light that brightened for 4 weeks and then faded, they report in today's issue of Nature. This pattern is consistent with a supernova explosion some 10 billion light-years away. The brightness would rise and fall as the debris from the star expands and cools, and because of the supernova's great distance, its light would be shifted toward red wavelengths by the expansion of the universe. A similar pattern in records of the aftermath of another gamma ray burst on 28 February 1997 has been reported in preprints by Titus Galama of Caltech and Daniel Riechart of the University of Chicago.

Although it's not yet clear how a supernova would produce gamma rays, theory suggests that black holes--which are thought to be formed in some supernovae--might throw off material in two jets. Powerful shock waves in the jets might generate the gamma rays, and electrons spiraling in magnetic fields associated with the jets might generate the lower energy afterglow. Only after the afterglow had faded would the telltale reddish light from the star's debris emerge. And when the gamma ray bursts strike, astronomers will be geared up to search out traces of supernovae.

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