October 5, 2005
Scientists have solved the 35-year-old mystery of the origin of powerful, split-second flashes of light known as short gamma-ray bursts. These flashes, brighter than a billion galaxies, yet lasting only a few milliseconds, have been simply too fast to catch - until now.
Through the unprecedented coordination of observations from several ground-based telescopes and NASA satellites, scientists determined the flashes arise from violent collisions in space. The clashes are either between a black hole and a neutron star or between two neutron stars. In either scenario, the impact creates a new black hole.
In at least one burst, scientists saw tantalizing, first-time evidence of a black hole eating a neutron star. The neutron star was first stretched into a crescent, then swallowed by the black hole.
Two recently detected bursts are featured in four papers in this week's Nature magazine. These observations could enable direct detection of exotic gravitational waves that have never before been seen.
"Gamma-ray bursts in general are notoriously difficult to study, but the shortest ones have been next to impossible to pin down," said Dr. Neil Gehrels, principal investigator for the Swift satellite at NASA's Goddard Space Flight Center, Greenbelt, Md. "All that has changed. We now have the tools in place to study these events," he said.
Gamma-ray bursts, first detected in the 1960s, are the most powerful explosions known. They are random, fleeting and can occur from any region of the sky. Two years ago, scientists discovered longer bursts, lasting more than two seconds, arise from the explosion of very massive stars. About 30 percent of bursts are short and under two seconds.
The Swift satellite detected a short burst on May 9, and NASA's High-Energy Transient Explorer (HETE) detected another on July 9. The May 9 event marked the first time scientists identified an afterglow for a short gamma-ray burst, something commonly seen after long bursts.
"We had a hunch that short gamma-ray bursts came from a neutron star crashing into a black hole or another neutron star, but these new detections leave no doubt," said Dr. Derek Fox, assistant professor of Astronomy & Astrophysics at Penn State University, State College, Pa. Fox is lead author of one Nature report detailing a multi-wavelength observation.
Fox's team discovered the X-ray afterglow of the July 9 burst with NASA's Chandra X-ray Observatory. A team led by Jens Hjorth, a professor at the University of Copenhagen identified the optical afterglow using the Danish 1.5-meter telescope at the La Silla Observatory in Chile.
Fox's team continued studying the afterglow with NASA's Hubble Space Telescope and ground-based telescopes of the Carnegie Institution, the National Astronomical Observatory of Japan, and the National Radio Astronomy Observatory.
"The July 9 burst was like the dog that didn't bark," said Dr. George Ricker, HETE principal investigator at the Massachusetts Institute of Technology, Cambridge, and co-author of another Nature article. "Powerful telescopes detected no supernova as the gamma-ray burst faded, arguing against the explosion of a massive star. Also, the July 9 burst, and probably the May 9 burst, are located in the outskirts of their host galaxies, just where old merging binaries are expected," he added.
Mergers create gravitational waves, ripples in space-time predicted by Einstein but never directly detected. The July 9 burst was about 2 billion light-years away. A big merger closer to the Earth could be detected by the National Science Foundation's Laser Interferometer Gravitational-Wave Observatory (LIGO). If Swift detects a nearby short burst, scientists could go back and check the data with a precise time and location.
"This is good news for LIGO," said Dr. Albert Lazzarini, Data & Computing group leader at the California Institute of Technology LIGO Lab, Pasadena. "The connection between short bursts and mergers firms up projected rates for LIGO, and they appear to be at the high end of previous estimates. Also, observations provide tantalizing hints of black hole-neutron star mergers, which have not been detected before," he said.
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George Deutsch/Erica Hupp
Goddard Space Flight Center, Greenbelt, Md.
Chandra X-ray Center, Cambridge, Mass.