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Gamma Ray Burst May Have a Sequel

25 January 1999 7:00 pm
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Missed the latest gamma ray burst? Never mind: There's about to be a replay, or so astronomers hope. Last Saturday, one of the brightest of these mysterious blasts of gamma and x-rays triggered satellite detectors. Scientists think it looked so bright because the gravity of a galaxy between Earth and the source focused its radiation toward us. And that means a reprise could come shortly, with the arrival of radiation refracted toward Earth along a different, slightly longer route.

The Italian-Dutch satellite BeppoSAX and NASA's Compton Gamma Ray Observatory picked up the initial gamma burst Saturday. A mere 18 seconds later, a robotic telescope in New Mexico operated by the University of Michigan, the Los Alamos National Laboratory, and the Lawrence Livermore National Laboratory recorded a relatively bright new "star" at the burst position. Half a minute later, this "optical transient" had brightened 15-fold, enough to be easily visible in amateur telescopes.

The brightness and the visible glow suggested that the source of the burst must be nearby, an idea seemingly supported by the discovery of a faint galaxy at the burst position. The galaxy's redshift--a measure of its distance--was between 0.2 and 0.3, modest in the cosmic scheme. The optical counterpart of the burst, however, had a redshift of 1.6. At that distance, roughly 80% of the way across the visible universe, the explosion would have had to have a staggering 2.3 × 1054 ergs of energy to explain its brightness at Earth, calculates George Djorgovski of the California Institute of Technology in Pasadena. Those numbers are implausible, Djorgovski thinks; instead, he thinks the faint galaxy in the foreground or the cluster it belongs to has acted as a gravitational lens, increasing the apparent brightness of the distant burst by a factor of 10 to 50 or so. "It is the first such case found," he says.

If Djorgovski is right, astronomers may have a second chance to study the event. Depending on how the gravitational lens has warped the paths of the radiation, additional burst images could flash into view within days, weeks, or months, says Jens Hjorth of the University of Copenhagen in Denmark. He hopes that this time around his colleagues will be ready to watch the burst from the moment it occurs.

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