03/19/2003 08:00:00

Quick action by astronomers worldwide leadsto new insights on mysterious gamma-ray bursts

Scientists "arriving quickly on the scene" of an October 4 gamma-ray burst have announced that their rapid accumulation of data has provided new insights about this exotic astrophysical phenomenon. The researchers have seen, for the first time, ongoing energizing of the burst afterglow for more than half an hour after the initial explosion.

The findings support the "collapsar" model, in which the core of a star 15 times more massive than the sun collapses into a black hole. The black hole's spin, or magnetic fields, may be acting like a slingshot, flinging material into the surrounding debris.

The prompt observation—and by far the most detailed to date—was made possible by several ground- and space-based observatories operating in tandem. The blast was initially detected by NASA's High-Energy Transient Explorer (HETE) satellite, and follow-up observations were quickly undertaken using ground-based robotic telescopes and fast-thinking researchers around the globe. The results are reported in the March 20 issue of the journal Nature.

"If a gamma-ray burst is the birth cry of a black hole, then the HETE satellite has just allowed us into the delivery room," said Derek Fox, a postdoctoral researcher at the California Institute of Technology and lead author of the Nature paper. Fox discovered the afterglow, or glowing embers of the burst, using the Oschin 48-inch telescope located at Caltech's Palomar Observatory.

Gamma-ray bursts shine hundreds of times brighter than a supernova, or as bright as a million trillion suns. The mysterious bursts are common, yet random and fleeting. The gamma-ray portion of a burst typically lasts from a few milliseconds to a couple of minutes. An afterglow, caused by shock waves from the explosion sweeping up matter and ramming it into the region around the burst, can linger for much longer, releasing energy in X rays, visible light, and radio waves. It is from the studies of such afterglows that astronomers can hope to learn more about the origins and nature of these extreme cosmic explosions.

This gamma-ray burst, called GRB021004, appeared on October 4, 2002, at 8:06 a.m. EDT. Seconds after HETE detected the burst, an e-mail providing accurate coordinates was sent to observatories around the world, including Caltech's Palomar Observatory. Fox pinpointed the afterglow shortly afterward from images captured by the Oschin Telescope within minutes of the burst, and notified the astronomical community through a rapid e-mail system operated by NASA for the follow-up studies of gamma-ray bursts. Then the race was on, as scientists in California, across the Pacific, Australia, Asia, and Europe employed more than 50 telescopes to zoom in on the afterglow before the approaching sunrise.

At about the same time, the afterglow was detected by the Automated Response Telescope (ART) in Japan, a 20-centimeter instrument located in Wako, a Tokyo suburb, and operated by the Japanese research institute RIKEN. The ART started observing the region a mere 193 seconds after the burst, but it took a few days for these essential observations to be properly analyzed and distributed to the astronomical community.

Analysis of these rapid observations produced a surprise: fluctuations in brightness, which scientists interpreted as the evidence for a continued injection of energy into the afterglow, well after the burst occurred. According to Shri Kulkarni, who is the McArthur Professor of Astronomy and Planetary Science at Caltech, the newly observed energizing of the burst afterglow indicates that the power must have been provided by whatever object produced the gamma-ray burst itself.

"This ongoing energy shows that the explosion is not a simple, one-time event, but that the central source lives for a longer time," said Kulkarni, a co-author of the Nature paper. "This is bringing us closer to a full understanding of these remarkable cosmic flashes."

Added Fox, "In the past we used to be impressed by the energy release in gamma-rays alone. These explosions appear to be more energetic than meets the eye."

Later radio observations undertaken at the Very Large Array in New Mexico and other radio telescopes, including Caltech's Owens Valley Radio Observatory and the IRAM millimeter telescope in France, lend further support to the idea that the explosions continued increasing in energy. "Whatever monster created this burst just refused to die quietly," said D. A. Frail, co-author and a staff astronomer at the Very Large Array.

Fox and his colleagues relied on data from the RIKEN telescope, in Japan, and from the Palomar Oschin Telescope and its Near Earth Asteroid Tracking (NEAT) camera, an instrument that has been roboticized and is currently managed by a team of astronomers at JPL led by Steven Pravdo. The collaboration of the Caltech astronomers and the NEAT team has proven extremely fruitful for the global astronomical community, helping to identify fully 25 percent of the afterglows discovered worldwide since Fox retrofitted the telescope software for this new task in the autumn of 2001.

HETE is the first satellite to provide and distribute accurate burst locations within seconds. The principal investigator for the HETE satellite is George Ricker of the Massachussetts Institute of Technology. HETE was built as a "mission of opportunity" under the NASA Explorer Program, a collaboration among U.S. universities, Los Alamos National Laboratory, and scientists and organizations in Brazil, France, India, Italy, and Japan.

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Contact: Robert Tindol (626) 395-3631