Scientists race to observe fading gamma-ray burst
Posted: October 9, 2002

The top image shows what that region of the sky looked like before the burst. Photo credit: Palomar Digital Sky Survey. The bottom image shows the GRB afterglow, marked as "OT" for optical transient, but taken with a different telescope. Photo credit: NEAT/Palomar 48" Oschin Telescope. Friday's GRB was detected at 8:06 a.m. EDT by the detectors onboard HETE. Within 11 seconds it sent out a message worldwide of the burst detection and a location 38 seconds later. Within nine minutes scientists at the California Institute of Technology made observations followed by groups in Kyoto, Japan; England and other countries. The explosion, a flash of gamma rays lasted approximately 100 seconds but the afterglow may linger in low-energy light forms for days or weeks.
Scientists have seen the afterglow of a gamma-ray burst just nine minutes after the explosion, a result of precision coordination and fast slewing of ground-based telescopes upon detection of the burst by NASA's High-Energy Transient Explorer (HETE) satellite.

The quick turnaround has so far allowed scientists to determine a minimum distance to the explosion, which likely marks the creation of a black hole. Results continue to pour in, as nearly 100 telescopes in 11 countries have tracked the burst.

The burst was detected on Friday, Oct. 4, at 8:06 a.m. EDT. NASA's Hubble Space Telescope and Chandra X-ray Observatory observed the afterglow on the following day, and another Hubble observation is planned for later this week. These and other observations are providing valuable clues to the mysterious nature of gamma-ray bursts, the most powerful explosions known.

"This is the big one that didn't get away," said George R. Ricker of the Massachusetts Institute of Technology in Cambridge, principal investigator for the international 20- person HETE team. "HETE sent out a burst alert in 11 seconds and then followed-up with an accurate location just 48 seconds later, while the bright gamma-ray emission was still in progress. HETE's prompt localization has resulted in this burst being by far the best-observed burst in the 30-year history of gamma-ray burst astronomy."

The burst lasted approximately 100 seconds, a relatively bright and long-lasting burst. Racing the clock and the break of dawn, Derek Fox, an astronomer at California Institute of Technology in Pasadena, turned the 48-inch Oschin Schmidt telescope at the Palomar Observatory to the location that HETE provided. Just nine minutes after the burst, Fox detected a fading, 15th-magnitude source -- the afterglow of the burst.

Gamma-ray bursts have the energy of a billion trillion Suns. Scientists have been hard-pressed to determine their origin, because they occur randomly in the universe and disappear quickly, usually within a minute or less. Theorists say the bursts are the creation of a black hole as a result of massive star explosions or the merger of neutron stars, or both.

HETE is designed to detect gamma-ray bursts and relay their locations within seconds to a worldwide network of radio, optical and X-ray telescopes. While the burst itself -- a flash of gamma rays, the most energetic form of light -- disappears quickly, the afterglow may linger in lower-energy light forms for days or weeks.

The optical afterglow of this burst is still so bright that it outshines the entire galaxy in which it is located, making it too bright to obtain information about its host galaxy for now.

Japanese astronomers in Kyoto and Bisei, under a blanket of dark sky, confirmed the Palomar observation and watched the burst's brightness fade by half over the next two hours. Seven hours after the burst occurred, astronomers at the Siding Spring Observatory in Australia reported the burst occurred more than 10 billion light-years from Earth.

By Saturday, amateur astronomers were also observing the spectacle. And in the hours and days to come, astronomers will comb the burst region with radio, X-ray and other optical telescopes, searching for more clues to the burst's origin.

HETE, a U.S. collaboration with France and Japan, is the first satellite dedicated to the study of gamma-ray bursts and is on an extended mission until 2004. NASA's Swift mission, planned for an October 2003 launch, is expected to detect, locate and observe bursts with even greater precision.

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