For the first time, scientists — racing the clock —
have snapped a photo of an unusual type of gamma-ray-burst
event one minute after the explosion. They captured a
particularly fast-fading type of “dark” burst, which
comprises about half of all gamma-ray bursts.

A gamma-ray burst announces the birth of a new black hole. It
is the most powerful type of explosion known, second only to
the Big Bang in total energy release. This latest finding may
double the number of gamma-ray bursts available for study and
rattle a few theories as well.

These “dark” bursts are so named because they have had no
detectable optical afterglow until now. Other bursts have
afterglows that linger for days or weeks, likely caused by
the explosion’s shock waves ramming into and heating gas in
the interstellar medium.

“Perhaps none of these bursts is truly ‘dark,’ provided we
can catch the burst fast enough,” said Dr. George Ricker of
the Massachusetts Institute of Technology (MIT) in Cambridge.
Ricker leads the international team that built and operates
NASA’s High Energy Transient Explorer (HETE), which
discovered the burst.

The orbiting HETE, which alerts scientists to gamma-ray
bursts, spotted one December 11, originating six billion
light-years away, and relayed its location to observatories
worldwide in 22 seconds. The ground-based RAPTOR (RAPid
Telescopes for Optical Response) optical telescope, operated
by the Los Alamos National Laboratory in New Mexico, was the
first on the scene, observing the afterglow at 65 seconds.
Other telescopes rushed to the event in the minutes that
followed.

The afterglow was gone in two hours and would have been
missed and labeled “dark” if not for HETE’s rapid turnaround.
Also, as chance would have it, this burst falls into a
subcategory of rare “transitional” bursts, between the short-
and long-duration variety, lasting only 2.5 seconds. Thus,
scientists have their most detailed look yet at the rarest of
gamma-ray bursts.

Gamma-ray bursts are common, yet random, and fleeting events
that have mystified astronomers since their discovery in the
late 1960s. Many scientists say longer bursts (over four
seconds) are caused by massive star explosions; shorter
bursts (under two seconds) are caused by mergers of binary
systems with black holes or neutron stars. While uncertainty
remains, most scientists say in either scenario a new black
hole is born.

Some theorists have suggested “dark” bursts have no
detectable afterglow because they are buried in thick dust
and gas, which blocks the afterglow’s light from reaching us.
Yet the new observation of the December 11 burst implies the
opposite. Ricker said the burst may have occurred in a region
with hardly any surrounding gas and dust; thus the shock
waves had little material to smash into to create a prolonged
afterglow.

In this case, the rapidly fading afterglow may support the
binary-merger theory of short bursts. Binary systems with a
combination of neutron stars or black holes are old, and in
the billions of years they take to form often work their ways
outward to less dense regions of a host galaxy. Thus, when
they merge, there is no material to make a long afterglow.

After HETE’s initial alert, Drs. Paul Price and Derek Fox of
the California Institute of Technology in Pasadena, Calif.,
were the first to report on the burst location using the 48-
inch Oschin Schmidt telescope at the Palomar Observatory in
California about 20 minutes after the burst. Reports are
posted on the publicly accessible Gamma-ray Burst Coordinates
Network Web site, operated by NASA’s Goddard Space Flight
Center in Greenbelt, Md., at:
http://gcn.gsfc.nasa.gov/

Later came reports of three earlier observations, with
RAPTOR, the Katzman Automatic Imaging Telescope (University
of California, Berkeley) and SuperLotis at Kitt Peak,
operated by Lawrence Livermore National Laboratory in
Berkeley, Calif.

Unraveling the gamma-ray burst mystery will require more
burst observations. HETE is pioneering a larger mission
called Swift, which NASA plans to launch in December 2003 to
make such observations routine.

HETE was built by MIT as a mission of opportunity under the
NASA Explorer Program. HETE is a collaboration of U.S.
universities, Los Alamos National Laboratory, and scientists
and organizations in Brazil, France, India, Italy and Japan.

For images and more information refer to:
http://www.gsfc.nasa.gov/topstory/2002/1223hetegrb.html

HETE: http://space.mit.edu/HETE

Additional images and GRB021004 information: http://space.mit.edu/HETE/Bursts/GRB021211