NASA'S HETE SPOTS RARE GAMMA-RAY BURST AFTERGLOW

Dolores Beasley
Headquarters, Washington             Nov. 7, 2001
(Phone: 202/358-1753)

Nancy Neal
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 301/286-0039)

Deborah Halber
Massachusetts Institute of Technology, Cambridge
(Phone: 617/258-9276)

RELEASE: 01-218

NASA'S HETE SPOTS RARE GAMMA-RAY BURST AFTERGLOW

     A rare optical afterglow of a gamma-ray burst, the most 
powerful type of explosion in the universe, was recently 
discovered by NASA's High Energy Transient Explorer (HETE), 
the first satellite dedicated to spotting these frequent yet 
random explosions that last only for a few seconds. 

The burst occurred in the constellation Lacerta, and was 
relatively close, only about 5 billion light-years from 
Earth. Gamma-ray bursts (GRBs) can be more than 10 billion 
light-years distant. "With this first confirmed observation 
of a gamma-ray burst and its afterglow, we've really turned 
the corner," said Dr. George Ricker of the Massachusetts 
Institute of Technology (MIT), Cambridge, principal 
investigator for HETE. "As HETE locates more of these bursts, 
we will begin to understand what causes them."  

The opportunity to see the afterglow in optical light 
provides crucial information about what is triggering these 
mysterious bursts, which scientists speculate to be the 
explosion of massive stars, the merging of neutron stars and 
black holes, or possibly both. 

The burst occurred Sept. 21, but because the enigmatic bursts 
disappear so quickly, scientists can best study the events by 
their afterglow. HETE detects these bursts as gamma rays or 
high-energy X-rays, and then instantly relays the coordinates 
to a network of ground-based and orbiting telescopes for 
follow-up searches for such afterglows. 

While GRBs often produce corresponding outpourings of X-rays, 
astronomers rarely detect visible light associated with the 
bursts, perhaps because they originate in regions of dense 
gas and dust that obscure any visible light that may be 
produced by the explosion. 

Additional observations of this event, made with the Italian 
BeppoSAX satellite and the Ulysses space probe, were 
coordinated by HETE team member Dr. Kevin Hurley at the 
University of California, Berkeley. The combination of the 
localization by this Interplanetary Network with the original 
HETE coordinates provided the refined information needed by 
ground-based observers to point their optical telescopes. 

Armed with the satellite-derived localization, the team led 
by Dr. Shri Kulkarni of the California Institute of 
Technology (Caltech), Pasadena, spotted the afterglow in 
optical light, with a Large Format Camera on the Palomar 200-
inch telescope on Sept. 22. In follow-up observations on Oct. 
17, the Caltech group measured the "redshift," the distance, 
of the afterglow object using the Double Spectrograph on the 
Palomar 200-inch telescope. In addition, on Oct. 17 they also 
pinpointed a twinkling radio counterpart using the Very Large 
Array in Socorro, N.M. "We believe that this object is very 
likely the afterglow of GRB 010921, detected and localized by 
HETE," said Kulkarni. 

The event was also captured 22 hours after the HETE trigger 
by a robotic telescope in Tucson, Ariz., operated by Dr. Hye-
Sook Park of the Lawrence Livermore National Laboratory, 
Livermore, Calif., and her colleagues. 

HETE was launched into near-Earth orbit Oct. 9, 2000, to 
detect gamma-ray bursts, which signal the extragalactic 
release of as much power as a billion trillion Suns. No one 
is sure though what causes them or exactly where they 
originate. Like beacons from the early universe, these bursts 
are thought to originate billions of light-years away.

"Gamma-ray bursts are the most energetic events since the Big 
Bang, yet one occurs about once a day somewhere in the sky," 
Ricker said. "The unique power of HETE is that it not only 
detects a large sample of these bursts, but it also relays 
the accurate location of each burst in real time to ground-
based optical and radio observatories."

HETE was built by MIT and is under NASA's Explorer Program, 
which is managed by NASA's Goddard Space Flight Center, 
Greenbelt, Md. HETE is a collaboration among NASA, MIT, Los 
Alamos National Laboratory, N.M.; France's Centre National 
d'Etudes Spatiales, Centre d'Etude Spatiale des Rayonnements, 
and Ecole Nationale Superieure de l'Aeronautique et de 
l'Espace; 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. HETE, the first satellite dedicated to the study of 
gamma-ray bursts, is on an extended mission until 2004.

More information on HETE can be found at: 
         http://space.mit.edu/HETE