Optical counterpart found for HETE-detected gamma-ray burst

WOODS HOLE, Mass. — 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 (HETE-2),
researchers from the Massachusetts Institute of Technology report
today in Woods Hole, Mass.

The MIT-built 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 are being announced today 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 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 of
MIT’s Center for Space Research, 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 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
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

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, 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 (Caltech),
Pasadena, Calif., 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, or distance, of the afterglow object using the Double
Spectrograph on the Palomar 200-inch 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

HETE-2 was built by MIT as a mission of opportunity under NASA’s
Explorer Program. The Explorer Program is managed by NASA Goddard
Space Flight Center, Greenbelt, Md. HETE is a collaboration between
NASA, MIT, Los Alamos National Laboratory, New Mexico; 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), 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, Joel Villasenor; at Los Alamos National
Laboratory, team members are Edward E. Fenimore, Mark Galassi, and
Tanya Tavenner; at the University of California at Berkeley, team
members are Kevin Hurley and J. Garrett Jernigan; at the University
of California at Santa Cruz, Stanford E. Woosley; at the University
of Chicago, team members are Don Lamb, Carlo Graziani, and Tim
Donaghy; and NASA project scientist at Goddard Space Flight Center in
Greenbelt, MD, is Thomas L. Cline.

HETE-2, 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