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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.

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