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09/17/1997 07:00:00

Astronomers Detect Relativistically Expanding Clouds Around the May 8 Gamma-Ray Burst

PASADENA—Astrophysicists still don't know what caused the gamma-ray burst of May 8, but they now have a size and rate of expansion for its remnant "fireball" to add to the location and distance.

New measurements by researchers at the California Institute of Technology and the National Radio Astronomy Observatory (NRAO) indicate that the fireball is now 85 times larger in diameter than our own solar system.

Further, the researchers have determined that the fireball is expanding at an extremely high rate of speed—perhaps as fast as 99.99 percent of the speed of light when the explosion first occured, and currently about 85 percent of light speed.

"This has all helped us to understand the mechanics of gamma-ray bursts once they take place," said Shrinivas Kulkarni, a Caltech astrophysicist who is coprincipal investigator of the work, which is reported in the September 18 issue of the journal Nature.

A few months ago the Caltech team, using the Palomar and the Keck telescopes, decisively showed that this gamma-ray burst occurred in a distant galaxy, settling one of the major controversies about the origin of these enigmatic objects.

Coprincipal investigator Dale Frail of NRAO added, "If you ask me what caused the burst, I'd still have to say it's pure speculation. It could have been a black hole smothering a neutron star, or maybe two neutron stars colliding, or perhaps even two black holes colliding.

"What we do know is that this was a spectacular cosmic event—far more energetic than a supernova explosion."

Gamma-ray bursters were first discovered by military satellites almost 30 years ago. The field has advanced rapidly, thanks to the precise localization of the bursts offered by the Italian-Dutch satellite BeppoSAX. Astrophysicists have now found, rapidly in succession, that gamma-ray bursts occur at cosmological distances and are probably the most energetic events in the universe.

This particular burst was first reported by BeppoSAX on May 8, 1997. Before the advent of BeppoSAX, astrophysicists had no idea whether gamma bursts originated in our own galaxy or across the universe, and, in fact, had formulated competing theories accounting for either scenario.

The measurements were obtained at the Very Large Array, a radio telescope array operated by NRAO with funding from the National Science Foundation. Other authors of the paper include Greg Taylor of NRAO and two BeppoSAX team members, Luciano Nicastro and Marve event."

Written by Robert Tindol