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SCIENTISTS
WORLDWIDE RACE TO OBSERVE FADING GAMMA-RAY BURST
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Arrow
indicates Gamma Ray Burst (above)
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Images
1 and 2
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Area
of sky before the burst
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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.
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Image
2
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HETE
observing gamma ray bursts - Click on image to view
animation.
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"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.
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