Satellite detects optical afterglow of gamma-ray burst

A rare optical afterglow of a gamma-ray burst, the most powerful type of explosion in the universe, was recently discovered by the High-Energy Transient Explorer satellite, MIT researchers reported last week.

The MIT-built High-Energy Transient Explorer (HETE-2) is the first satellite dedicated to spotting gamma-ray bursts (GRBs)--frequent, random explosions that last for only a few seconds. The latest results were announced Nov. 7 at an international conference, "Gamma-Ray Burst and Afterglow Astronomy 2001: A Workshop Celebrating the First Year of the HETE Mission," held in Woods Hole, Mass., from Nov. 5-9.

GRBs can be more than 10 billion light years distant, but this one occurred in the constellation Lacerta, only around 5 billion light years from Earth.

"With this first confirmed observation of a gamma-ray burst and its afterglow, we've really turned the corner," said George Ricker, a senior research scientist at the Center for Space Research and principal investigator for HETE-2. "As HETE-2 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 are the explosions 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 afterglows. HETE-2 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 GRBs, 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 Kevin Hurley at the University of California at 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 Shri Kulkarni of the California Institute of Technology 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 red shift, or distance, of the afterglow object using the double spectrograph on the Palomar telescope. In addition, they pinpointed a twinkling radio counterpart using the Very Large Array radio telescope in New Mexico. "We believe that this object is very likely the afterglow of GRB 010921, detected and localized by HETE," said Kulkarni.


HETE-2 was launched into near-Earth orbit Oct. 9, 2000 to detect GRBs, which signal the extragalactic release of as much power as a billion trillion suns, but no one is sure 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-2 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-2 was built by MIT as a mission of opportunity under NASA's Explorer Program, which is managed by NASA Goddard Space Flight Center. HETE is a collaboration between NASA; MIT; Los Alamos National Laboratory; 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.

At MIT, the HETE-2 team includes Ricker, Nat Butler, Geoffrey Crew, John Doty, Allyn Dullighan, Steve Kissel, Alan Levine, Francois Martel, Fred Miller, Glen Monnelly, Ed Morgan, Gregory Prigozhin, Roland Vanderspek and Joel Villasenor.

HETE-2, the first satellite dedicated to the study of gamma-ray bursts, is on an extended mission until 2004.

A version of this article appeared in MIT Tech Talk on November 14, 2001.

Topics: Space, astronomy and planetary science

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