Big Radio From the Stars

Astronomers have a new mystery on their hands. A team studying data from a recent sky survey has spotted a huge burst of radio waves that came and went in the blink of an eye and has not returned since. Figuring out what caused the cosmic broadcast could provide new insights into the dynamics of neutron stars and black holes and perhaps give astronomers another tool to figure distances to the farthest galaxies.

Radio waves come in all strengths across the universe, from the extremely faint signals sent by distant spacecraft such as the Voyagers and Pioneers to the extremely powerful waves generated by rapidly spinning neutron stars called pulsars. Even diffuse clouds of hydrogen gas floating between galaxies can create them. The mystery signal represents something else entirely. It packed as much energy in its mere 5-millisecond duration as the sun puts out in a month, making it by far the strongest, quickest signal radio astronomers have observed, although it wasn't nearly as powerful as the elusive gamma ray bursts that populate the universe.

The signal was so unexpected astronomers didn't even know where it came from. At first glance, it appeared to have originated in an area of the sky around the Magellanic Clouds, two small satellite galaxies orbiting the Milky Way at a distance of about 200,000 light-years. Upon closer examination of the data--compiled from nearly 500 hours of observation by the 64-meter Parkes radio telescope in Australia--a team led by astronomer Duncan Lorimer of West Virginia University in Morgantown estimated that the blast actually came from about 3 billion light-years away. It turns out that the signal's component high-frequency and low-frequency elements were separated slightly by a phenomenon called dispersion, caused when radio waves travel through ionized gas clouds in intergalactic space. The team thinks dispersion could be used to help compute cosmic distances with greater precision--if more radio bursts can be detected and their cause can be isolated.

Reporting online today in Science, the researchers say they think only two phenomena could be behind the radio burst they discovered: the merger of two neutron stars or the final evaporation of a black hole. NASA astrophysicist Valerie Connaughton of the University of Alabama, Huntsville, isn't sure either hypothesis will hold up, because no radio burst has ever been associated with either phenomenon. If this one can be linked, however, it would be a "huge deal," she says.

Whatever caused the signal, "it's bound to be exciting," says radio astronomer Lawrence Rudnick of the University of Minnesota, Minneapolis. The source is "almost certainly a very high-energy phenomenon," he says, that is bound to "push us into exciting new realms of physics."

Lorimer says his team next plans to reexamine the Parkes data, as well as other archives of radio signals, to see if similar bursts turn up. "We're also designing experiments to stare at as much of the sky as we can for as long as we can," he says. One possible instrument would be the Allen Telescope Array in California, Lorimer says, which has a much wider field of view than the Parkes instrument.

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