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Scientists "arriving quickly on the scene" of a
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.
Derek Fox, a postdoctoral researcher at the California Institute
of Technology and lead author of the Nature paper, 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.
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.
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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