MIT Press Release on GRB021211

Scientists catch their first elusive "dark" gamma-ray burst

CAMBRIDGE, Mass.--Scientists racing the clock have snapped a photo of 
a gamma-ray burst event one minute after the explosion, capturing for 
the first time 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, said scientists from the Massachusetts Institute of Technology, 
based on an X-ray image taken by the MIT-built High Energy Transient 
Explorer (HETE) satellite, the first satellite dedicated to spotting 
gamma-ray bursts.

These dark bursts are so named because they have had no detectable 
optical afterglow, until now. Other bursts have bright 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 that we catch 
them fast enough," said George Ricker, a senior research scientist at 
MIT's Center for Space Research, who leads the international team 
that built and operates NASA's HETE satellite.

The orbiting HETE, which alerts scientists to gamma-ray bursts, 
spotted one on Dec. 11 originating six billion light years away and 
relayed its location to observatories worldwide in 22 seconds. The 
ground-based Raptor optical telescope, operated by the Los Alamos 
National Laboratory, 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 extremely faint after 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, in 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 that longer bursts (lasting more than 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 that in 
either scenario, a new black hole is born.

Some theorists have suggested that 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 Dec. 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 bright afterglow."

The rapidly fading afterglow, in this case, 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 took 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, Paul Price and Derek Fox of Caltech were 
the first to report on the burst location using the 48-inch Oschin 
Schmidt telescope at the Palomar Observatory about 20 minutes after 
the burst. Reports are posted on the publicly accessible Gamma-ray 
Burst Coordinates Network web site, operated by NASA Goddard Space 
Flight Center in Greenbelt, Md. Later came reports of three earlier 
observations, with RAPTOR (RAPid Telescopes for Optical Response), 
the Katzman Automatic Imaging Telescope (University of California at 
Berkeley) and SuperLotis (Lawrence Livermore National Laboratory at 
Kitt Peak).

HETE was built by MIT as a mission of opportunity under the NASA 
Explorer Program. It is on an extended mission until 2004. The HETE 
program is a collaboration between MIT; NASA; Los Alamos National 
Laboratory, New Mexico; France's Centre National d'Etudes Spatiales 
(CNES), Centre d'Etude Spatiale des Rayonnements (CESR), and Ecole 
Nationale Superieure del'Aeronautique et de l'Espace (Sup'Aero); 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 team includes Ricker, Geoffrey Crew, John Doty, 
Roland Vanderspek, Joel Villasenor, Nat Butler, Allyn Dullighan, Glen 
Monnelly, Gregory Prigozhin, Steve Kissel, Alan Levine, Francois 
Martel, Fred Miller; 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.

For more information:

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

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