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Gamma-ray bursts: Going the distance.

Astronomers may have identified a new and quicker way to determine the distance to those mysterious flashes of energetic radiation known as gamma-ray bursts. If the finding proves correct, scientists could use bursts to probe some of the earliest events in the cosmos.

Although Earth-orbiting satellites record a burst popping off at least once a day, astronomers have measured the distance to only a handful. That's because scientists must find the fleeting, visible-light counterpart to the gamma rays and then determine its redshift--the amount by which the expansion of the universe has shifted the light to redder, or longer, wavelengths.

Two researchers have now closely examined the six bursts for which astronomers have measured distances. Enrico Ramirez-Ruiz, now at the University of Cambridge in England, and Edward E. Fenimore of the Los Alamos (N.M.) National Laboratory calculated the intrinsic brightness of each flash. They found that the most luminous flashes flicker the most, while those that are intrinsically dimmest vary the least.

If that pattern holds true, scientists would have a new way to determine distance. They would simply compare the luminosity of a burst, as indicated by the flickering, with the observed brightness, which declines as the inverse square of distance.

Ramirez-Ruiz and Fenimore presented their findings last month at the annual gamma-ray-burst symposium in Huntsville, Ala. If the link between flickering and brightness proves reliable, a burst could serve as a new yardstick for measuring distances to other objects. Because bursts are bright enough to be detected from far away, the yardstick would allow astronomers to measure distances deep in space and far back in time, Fenimore says.

"The correlation looks good, but there are still very few data points," says Stan Woosley of the University of California, Santa Cruz.

At the conference, Jay P. Norris of NASA's Goddard Space Flight Center in Greenbelt, Md., presented evidence for another type of distance indicator. Norris finds that the more luminous the measured burst, the smaller the lag time between its high-energy and low-energy photons.
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Title Annotation:determining distance in the universe
Publication:Science News
Article Type:Brief Article
Date:Nov 13, 1999
Words:336
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