An international team of astronomers has discovered a new class of stars--massively compressed old neutron stars that seem inactive but for intermittent bursts of radio waves. Dubbing them rotating radio transients (RRATs), the researchers note that their isolated outbursts last for as few as two milliseconds and are separated by gaps as long as three hours.

"These things were very difficult to pin down," says Dick Manchester of the Commonwealth Scientific and Industrial Research Organization's (CSIRO) Australia Telescope National Facility. "For each object we've been detecting radio emissions for less than one second a day. And because these are single bursts, we've had to take great care to distinguish them from terrestrial radio interference."

The researchers first found 11 such objects in a search for isolated radio signals in data recorded during a four-year survey of a slice of the night sky by the CSIRO Parkes radio telescope. The survey set out to document pulsars--neutron stars that spin and emit powerful radio beams--and found a host of them, along with these strange objects. Since August 2003, each RRAT has been observed at least nine more times to detect multiple bursts. Despite their intermittent nature, the bursts make these new stars among the strongest radio sources in the universe.

The astronomers' analysis suggests that these neutron stars rotate like more regular pulsars, but only pulse occasionally. If these stars belonged to binary systems, as some other pulsars do, their reappearance from behind their partners might explain the periodic bursts. The data seem to show that they do not, however.

Given the fact that the single, 35-minute observation that first revealed these new stars had only a roughly 20 percent chance of catching one of these outbursts, there may be as many as five times more of these RRATs than the 100,000 or so constant pulsars in the Milky Way. That makes these temperamental pulsars a more common member of the universal star population than their more widely known cousins. "This discovery increases the current galactic population estimates [for pulsars] by at least several times," the team writes in their report on the findings, published today in Nature. "We therefore expect the emerging generation of wide-field radio telescopes to discover many more RRATs."

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