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 the High-Energy Transient Explorer (HETE) satellite developed by an MIT-led international team.
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, Oct. 4 at 8:06 a.m. EDT. Follow-up observations, including those quickly scheduled with NASA's Chandra X-ray Observatory and Hubble Space Telescope, are providing valuable clues to the mysterious nature of the gamma-ray burst, an example of the most powerful explosions known.
"This is the big one that didn't get away," said George R. Ricker, senior research scientist at MIT's Center for Space Research and 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-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. HETE is crucial in notifying telescopes where to look for the afterglow, which contains information about the burst's origin.
Currently, the optical afterglow of burst GRB021004 is still so bright that it outshines the entire galaxy in which it is located. This is too bright to attain information about its host galaxy, according to Shri Kulkarni of the California Institute of Technology, who leads an effort there to understand gamma-ray bursts. Over the next month, as the afterglow fades, the faint host galaxy will become visible and can be studied in detail.
Burst GRB021004 lasted approximately 100 seconds, a relatively bright and long burst. Racing the clock and the break of dawn, Derek Fox, a Caltech astronomer, 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. The afterglow lay neatly within the 1-arcminute location region derived by HETE's soft X-ray camera (SXC), clinching the identification.
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, a Caltech-Cambridge-Sydney collaboration at the Siding Spring Observatory in Australia reported an absorption redshift of 1.6, roughly equivalent to a distance of 15 billion light years.
The measurement is based on material - perhaps a galaxy or cloud of gas - that absorbed some of the burst's light as it passed through on its long journey to Earth. This means that the burst occurred beyond the absorption region, at 15 billion light years distance or greater.
By Saturday, amateur astronomers - tuned to the Gamma-ray Burst Coordinates Network - were also observing the spectacle. 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 was built by MIT as a mission of opportunity under the NASA Explorer Program. The HETE program is a collaboration between MIT, NASA, Los Alamos National Laboratory, the Centre National d'Etudes Spatiales, the Centre d'Etude Spatiale des Rayonnements and the Ecole Nationale Superieure del'Aeronautique et de l'Espace (all in France), and Japan's Institute of Physical and Chemical Research (RIKEN). The science team includes members from the University of California (Berkeley and Santa Cruz) and the University of Chicago, as well as from Brazil, India and Italy.
At MIT, the HETE-2 team includes Ricker, Nat Butler, Geoffrey Crew, John Doty, Allyn Dullighan, Steve Kissel, Alan Levine, Francois Martel, Fred Miller, Glen Monnelly, Gregory Prigozhin, Roland Vanderspek and Joel Villasenor.
HETE, the first satellite dedicated to the study of gamma-ray bursts, is on an extended mission until 2004.
A version of this article appeared in MIT Tech Talk on October 9, 2002.