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Vanishing Galaxy May Be Source of Far-Out Gamma Rays

UTRECHT, THE NETHERLANDS--Dutch astronomers have linked a massive burst of gamma rays last month to a distant galaxy that grew much dimmer afterward. The observation, described in an astronomical bulletin late last week, is the strongest evidence yet that such mysterious discharges originate far beyond the Milky Way.

Ever since gamma-ray bursts were first detected in the 1960s, astronomers have debated whether the mysterious, high-energy flashes are the products of violent events on the surfaces of neutron stars close to our galaxy or of even more violent processes in the universe's farthest reaches, such as collisions between neutron stars or black holes. But neither idea could win out, because astronomers had never been able to link a burst to an identifiable object.

On 28 February, the Italian-Dutch satellite BeppoSAX detected a burst of gamma rays from the northwestern part of the constellation Orion. The satellite traced the burst to a source of x-rays that dwindled afterward, enabling astronomers to pinpoint the burst's location to less than an arc minute, roughly a thirtieth of the full moon's width (Science, 14 March, p. 1560). Just 21 hours later, Jan van Paradijs and his colleagues at the University of Amsterdam trained two optical telescopes at the Roque de los Muchachos Observatory in the Canary Islands on the same region of sky. A first pass failed to turn up anything unusual in the clutter of galaxies there. On 8 March, however, van Paradijs's group saw that one of the bright spots had grown much dimmer. Five days later, van Paradijs's group and astronomers at the European Southern Observatory found that a small galaxy had performed the vanishing act.

To van Paradijs and his colleagues, the observations suggest--but don't prove--that the burst came from this distant galaxy. "It smells good," he says. But if bursts do originate in the nether regions of the universe, the next question is what causes them. More efforts like this one, combining observations from gamma detectors, x-ray cameras, and optical telescopes, "may finally put constraints on the many wild theories" of gamma-ray bursts, says Frank Verbunt of the University of Utrecht.