First elusive 'dark' gamma-ray burst caught by scientists
Posted: December 23, 2002

The High Energy Transient Explorer detected the 2.5 second gamma-ray burst and relayed its location to telescopes worldwide in 22 seconds. The RAPTOR ground-based telescope caught the afterglow 65 seconds later, on the edge of a galaxy several billion light years away. Pictured here: the afterglow fading fast at about 65 seconds (left image), then 9 minutes (right). The afterglow was undetectable about two hours later. Credit: P. Wozniak, W.T. Vestrand, et al., RAPTOR Project, LANL
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:

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.

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