10/05/2005. Contributed by Jessica Martin
NASA scientists have, for the first time, detected and pinned down the location of a short gamma-ray burst, lasting only 50 milliseconds. The burst marks the birth of a black hole. The astronomy community is speculating on what may have caused the burst; perhaps a collision of two older black holes or two neutron stars.
Gamma-ray bursts are the most powerful explosions in the universe. Bursts lasting more than two seconds have been observed by NASA satellites such as Swift, built to detect and quickly locate the flashes. Short bursts had remained elusive until May 9, when Swift detected the recent flash called GRB 050509B. Swift autonomously locked onto the location, and the satellite focused its onboard telescopes in less than a minute to capture the burst afterglow.
"Seeing the afterglow from a short gamma-ray burst was a major goal for Swift, and we hit it just a few months after launch," said Dr. Neil Gehrels. He is the Swift project scientist at NASA's Goddard Space Flight Center (GSFC), Greenbelt, Md. "For the first time, we have real data to figure out what these things are," he added.
The afterglow contains information about what caused the burst. Most scientists are convinced short and long bursts arise from two different catastrophic origins. The longer bursts appear to be from massive star explosions in very distant galaxies. The shorter ones, less than two seconds and often just a few milliseconds, are the deeper mystery, because they have been too fast for detailed observation.
Swift's X-ray telescope detected a weak afterglow that faded away after about five minutes. Swift's Ultraviolet/Optical Telescope did not see an afterglow. Ground-based telescopes have not definitely detected an afterglow. In contrast, afterglows from long bursts linger from days to weeks.
All of this fits the pattern of a collision between some combination of black holes or neutron stars, both of which are created during the deaths of massive stars. Neutron stars are dense spheres about 20 miles across. Black holes have no surface and are regions in space of infinite density. Theories predict these collisions wouldn't produce a long afterglow, since there isn't much fuel, such as dust and gas, from the objects or the region to sustain one.
The burst appears to have occurred near a galaxy that has old stars and is relatively nearby, about 2.7 billion light years away from Earth. This is consistent with the theory that short bursts come from older, evolved neutron stars and black holes. In contrast, longer gamma-ray bursts tend to be in young, distant galaxies filled with young, massive stars, remnants of the early universe.
"We are combing the region around the burst with the Keck Telescope in Hawaii for clues about this burst or its host galaxy," said Shri Kulkarni, from the California Institute of Technology, Pasadena, Calif. Kulkarni is a gamma-ray burst expert. "What we are seeing so far is what proponents of the merger theory have been saying all along," Kulkarni added.
Additional observations are planned for NASA's Hubble Space Telescope and Chandra X-ray Observatory.
Swift is a NASA mission in partnership with the Italian Space Agency and the Particle Physics and Astronomy Research Council of the United Kingdom. Swift is managed by GSFC. Penn State University controls science and flight operations from the Mission Operations Center in University Park, Pa. The spacecraft was built in collaboration with national laboratories, universities and international partners, including Penn State; Los Alamos National Laboratory, N.M.; Sonoma State University, Rohnert Park, Calif.; Mullard Space Science Laboratory in Dorking, Surrey, England; the University of Leicester, England; Brera Observatory, Milan and ASI Science Data Center, Frascati, Italy.
For more information about Swift on the Internet, check out http://swift.gsfc.nasa.gov/
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