Astronomers detect 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 into the exotic astrophysical phenomenon. For the first time, researchers have seen 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 whose spin, or magnetic fields, may be flinging material like a slingshot into the surrounding debris.

Several observatories operating in tandem enabled the observation, by far the most detailed to date. The blast was initially detected by NASA’s High-Energy Transient Explorer (HETE) satellite, and fast-thinking researchers worldwide began using ground-based robotic telescopes. The results were reported in the March 20 issue of 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,” says Caltech postdoctoral scholar in astronomy Derek Fox, the paper’s lead author. He discovered the afterglow, or glowing embers of the burst, using the 48-inch Oschi Telescope at Caltech’s Palomar Observatory.

Gamma-ray bursts shine hundreds of times brighter than a supernova and, though common, are random and fleeting. The gamma-ray portion of a burst typically lasts from a few milliseconds to a couple of minutes. The 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 the form of X-rays, visible light, and radio waves. By studying such afterglows, astronomers hope to learn more about the origins and nature of these cosmic explosions.

This gamma-ray burst, called GRB021004, appeared on October 4 at 8:06 a.m., EDT. Seconds after HETE detected the burst, Palomar and observatories around the world received an e-mail with accurate coordinates. Fox pinpointed the afterglow soon afterward from images captured by the Oschin Telescope within minutes of the burst, and notified the astronomical community through NASA’s rapid e-mail system for following up such bursts. Then the race was on, as scientists around the globe employed more than 50 telescopes to zoom in on the afterglow before sunrise. At about the same time, the afterglow was detected by a telescope operated by the Japanese research institute RIKEN.

Analysis of the observations produced a surprise: fluctuations in brightness, which scientists interpreted as evidence for a continued injection of energy into the afterglow, well after the gamma-ray burst occurred.

“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 Shri Kulkarni, Caltech’s MacArthur Professor of Astronomy and Planetary Science and a study coauthor. “This is bringing us closer to a full understanding of these remarkable cosmic flashes.”

Fox and his team relied on data from RIKEN’s telescope and from the Oschin Telescope and its Near Earth Asteroid Tracking (NEAT) camera. The Caltech-NEAT collaboration has helped to identify fully 25 percent of afterglows discovered worldwide since autumn 2001.

The first satellite to provide and distribute accurate burst locations within seconds, HETE was built under the NASA Explorer Program, a collaboration between U.S. universities, Los Alamos National Laboratory, and scientists in Brazil, France, India, Italy, and Japan.

More details, including images and animation, can be found at http://www.gsfc.nasa.gov/topstory/2003/0319hete.html. Details on the study can be found at www.nature.com.
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