Half of the GRBs that scientists have observed in this manner, however, have no known optical or visible light component, and are so-called "dark" bursts. Scientists speculate that these dark GRBs may fade too quickly for telescopes to detect or are enshrouded in so much dust that the light is hidden.

The new observations, initiated by NASA's space-based High Energy Transient Explorer (HETE), point to the former theory, says George Ricker, a senior research scientist at the Massachusetts Institute of Technology and principal investigator on HETE.

"This burst seems to show that the first answer is right, that these things fade very, very fast," Ricker says. "The thing that happened with this particular burst is that it was caught within a minute after the thing went off. And even though it faded extremely fast, the fact that there was an accurate localization and an optical counterpart that had been determined when it was still bright, it meant that it could be followed to fainter magnitudes than had previously been the case."

HETE first detected the burst through its x-ray emissions on December 11 in a galaxy six billion light years away. Three optical telescopes in Arizona and California, which are linked to HETE and respond to its alerts, were automatically notified within 22 seconds. Within one minute, the first group of astronomers had successfully imaged the GRB.

"We would have been happy if one had just gotten it, but coincidentally three got it, which is pretty amazing," Ricker says. There are about seven telescopes worldwide that are linked to HETE's alerts, which are also coordinated through Goddard Space Flight Center.

GRBs were first detected in the 1960s. They can be separated into two categories: short bursts that last a second or less, and long bursts that last longer than four seconds. This event lasted about 2.5 seconds. "This burst is curious because it's right on the border-line between these two populations," Ricker says. "There's only 2 or 3 other bursts that are known to have durations that are this short. If it's a long burst it's at the very short end of the distribution; and if it's a short burst it's very long."

Preliminary data are consistent with this GRB falling into the short population, Ricker says. Astronomers believe that short bursts occur when two objects merge, either a black hole with a neutron star, or two neutron stars. Because these systems are older, they may travel out to the farther reaches of a galaxy, where there is less gas and dust. "When the explosion takes place, there would be very little material for the relativistic shock wave to light up and make an afterglow," Ricker says. "The afterglow might be very faint, or it might fade very rapidly. So that's consistent with what we're seeing in this one."

Ricker adds that data taken last week by the Hubble Space Telescope, which is currently being analyzed, will pinpoint the distance of the event from the center of its host galaxy. "The Hubble image will really tell the story in detail," Ricker says.

HETE was launched in October 2000 but has been in operation at its current levels of sensitivity for nine months. It has detected over 100 bursts, Ricker says, but this most recent GRB is the thirtieth that it has pinpointed with any accuracy. Another GRB was localized on October 4, but the network of automated telescopes was not yet in operation, so ground-based observatories did not image the event until nine minutes after it occurred.

HETE was built by MIT for NASA. It is operated by an international group of universities, organizations and scientists.

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