An artist's concept of gamma ray bursts with HETE-2 watching. Photo: NASA-GSFC

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.

An artist's concept of HETE-2. Photo: NASA-GSFC

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.