Gamma-Ray Astronomy: Repeated Bursts Puzzle Astronomers
Andrew Watson
Andrew Watson is a writer in Norwich, U.K.
Last week, a U.S.-Russian team announced that four space-based detectors have picked up what might be a vital clue in one of the biggest mysteries of modern astronomy--or a statistical fluke. In the space of 2 days in late October, the detectors, aboard spacecraft including the Compton Gamma-Ray Observatory (GRO) and Ulysses, recorded four gamma-ray spikes from the same small patch of sky. The spikes were immediately identified as signaling gamma-ray bursts--seconds-long blasts of gamma rays that come about once a day from random directions. No one knows what produces gamma-ray bursts, in part because none has ever been correlated with any visible source. And every burst to date has apparently occurred in splendid isolation.
The implication of the new observations--that a single source produced repeated bursts--is a startling one, say researchers. "How can something produce emission over a 2-day period?" asks team member Charles Meegan of NASA's Marshall Space Flight Center in Huntsville, Alabama. At the same time, the new observation may end up giving astronomers their first break in the case. It suggests, say some, that the bursts must come from the neighborhood of our galaxy and not, as other astronomers believe, from the fringes of the universe.
The researchers, who announced the observations in an International Astronomical Union publication on the Internet, are fairly certain that the first two short spikes, seen on 27 October, represent two distinct bursts. Not so the pair of spikes that came 2 days later, just 23 minutes apart. "We are convinced those two come from the same source and that it's one real long burst," says Meegan--the longest ever recorded.
A repeating gamma-ray burst might allow astronomers to eliminate some of the rival explanations for these events. One theory holds that bursts are caused by coalescing neutron stars in the distant universe, but that process "could not make something that would burst again 2 days later," says Meegan. And for any source to produce the intense bursts repeatedly, it cannot be far away, argues team member Bonnard Teegarden of NASA's Goddard Space Flight Center in Greenbelt, Maryland.
Then again, the multiple burst could be a coincidence: several different sources that happened to be in the same patch of sky. "It seems to be unlikely to be due to chance, although we still need to work real hard to make sure," says Meegan. But Ralph Wijers of Cambridge University's Institute of Astronomy has analyzed the numbers and has come to his own conclusions. "BATSE [a GRO instrument] has been observing the sky for five-and-a-half years, and you would in fact expect about one time in that whole list to have three bursts from roughly that size area of sky within a few days," he says. "If you go back and look at the hard numbers, it is not as remarkable as you might think."
