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See Inside June 2005

Rare Flare

Odd magnetar burst partly solves gamma-ray riddle
STAR WITH FLARE



NASA
It was the brightest cosmic explosion ever observed, and astronomers are still hotly debating its origin and implications. But already the giant flare of December 27, 2004, produced by a bizarre star in our own Milky Way galaxy, is providing a partial solution to a 10-year-old astrophysical mystery. Such "magnetar" flares in distant galaxies may account for at least some of a particular class of gamma-ray burst that has defied explanation.

Despite its distance of 50,000 light-years, the December flare was brighter than the full moon. Yet no one actually saw it, because it belched out almost all its stupendous power in the form of energetic gamma rays, completely saturating the sensitive Burst Alert Telescope on NASA's Swift satellite, which had been launched into orbit just five weeks earlier. "It was an astonishing event," recalls gamma-ray-burst researcher Ralph Wijers of the University of Amsterdam in the Netherlands.

After learning of the giant flare, Swift scientist David Palmer of Los Alamos National Laboratory immediately had a hunch. If a similar magnetar flare occurred in a distant galaxy, he reasoned, it would be indistinguishable from a so-called short gamma-ray burst, with a duration less than two seconds or so. These short bursts are quite different from their longer cousins, which last from a few seconds to many minutes. Astronomers believe that long gamma-ray bursts, all detected in remote galaxies so far, signal the catastrophic and terminal detonation of supermassive, rapidly spinning stars. This proposed mechanism probably does not apply to short gamma-ray bursts, however.

Palmer developed his idea and found that the magnetar flares offer at least a partial explanation. In an analysis to be published in Nature, he and his colleagues conclude that at least a few percent of all short bursts are quite likely to be explained in this way. Based on the observed luminosity and expected frequency of giant magnetar flares, a few dozen of these events per year would occur in other, relatively nearby galaxies. This amount is not enough to explain all short gamma-ray bursts, but, Palmer says, "5 percent is a good approximation." He quips that this number "is probably not off by more than a factor of 20, which is actually pretty good in this business."

As for the cause of the other short gamma-ray bursts, Chryssa Kouveliotou of the NASA Marshall Space Flight Center says that the leading explanation is the violent merger of two neutron stars orbiting each other. But Palmer notes: "With the December 27 event, we now know that neutron-star mergers are not responsible for all short gamma-ray bursts. Whether they are responsible for any of them is still an open question." Wijers agrees that it remains unclear whether a neutron-star merger can produce this type of gamma-ray burst.

The answer may come soon, though. Astronomers expect that the Swift satellite, which became fully operational in early April, will accurately pinpoint sky positions and distances for a number of short bursts, enabling scientists to finally get a grip on these enigmatic phenomena. Palmer, for one, is optimistic: "The next gamma-ray burst we see could bring enlightenment."

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