SCIENTISTS WORLDWIDE RACE TO OBSERVE FADING GAMMA-RAY BURST

by Goddard Space Flight Center

Scientists have captured the optical 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, named GRB021004, was detected on Friday, October 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 MIT, principal investigator for the 20-person international 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 GRB021004 being by far the best observed burst in the 30-year history of gamma-ray burst astronomy."

Gamma-ray bursts have the energy of a billion trillion suns, yet they occur randomly and disappear within a few seconds to about a minute. Thus, scientists have been hard-pressed to determine the origin of these events. Theorists say the bursts are a result of massive star explosions (larger than supernovae) 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-rays 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 afterglow contains information about the burst's origin.

The optical afterglow of burst GRB021004 is still so bright that it outshines the entire galaxy in which it is located, too bright to obtain information about its host galaxy for now.

Burst GRB021004 lasted approximately 100 seconds, a relatively bright and long burst. Racing the clock and the break of dawn, Derek Fox, an astronomer at California Institute of Technology, 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.

Japanese astronomers in Kyoto and Bisei, under a blanket of dark sky, confirmed the Palomar observation and watched the source fade over the next two hours by about a factor of two. Seven hours after the burst occurred, astronomers at the Siding Spring Observatory in Australia reported an absorption redshift of 1.6; this is distance measurement equivalent to over 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.