Astronomers think they've nailed down the source of a mysterious blast of gamma rays that reached Earth in late March and continues, at reduced levels, even today. The culprit looks like a black hole, 3.8 billion light-years away, that swallowed and ripped apart a wandering star. On March 28, NASA's Swift satellite first noted the outburst of invisible radiation, a gamma ray burst, one of the most powerful explosions in the universe. Such blasts, thought to result from the explosion of massive stars, are regularly detected and usually die away within minutes. But this one continues today, and in its first two days, the intensity of the outburst measured in some wavelengths not visible to the naked eye as bright as a hundred billion suns, scientists report in Thursday's edition of the journal Science. That makes it one of the most intense cosmic explosions ever witnessed by astronomers. "This is probably the first time mankind has seen a phenomenon like this," says astronomer Josh Bloom of the University of California, Berkeley, lead author of one of two studies on the outburst The finding adds to evidence that most galaxies, including our own Milky Way, likely harbor titanic black holes at their center, mostly quiet, but always waiting to pull part anything that wanders too close.

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