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