AMSTERDAM--Amidst the cosmic cacophony of radio hisses, murmurs, and chirps, a single bleep may easily go unnoticed. Little wonder then that a large population of "silent pulsars" could go undetected for decades. Now, an international team of astronomers has stumbled upon 11 of the mysterious objects, and there may be millions more. The find may help astronomers better understand the makeup of our Milky Way galaxy.
Normal pulsars (short for pulsating stars) are rapidly spinning neutron stars. These dense balls of nuclear matter, some 20 kilometers across, are the mortal remains of once-massive giant stars that ended their brief lives in titanic supernova explosions. If orientated properly, a hot spot above the magnetic pole of the neutron star may whirl in and out of view for observers on Earth, producing a regular train of radio pulses separated by anything from a few milliseconds to a few seconds. Although many pulsars sometimes miss a few pulses or emit a giant pulse, they are usually relatively easy to spot because they're pulsing anytime someone's looking.
Not so with the new class of pulsars that Andrew Lyne of Jodrell Bank Observatory, United Kingdom, and his team found in data obtained with the Parkes Multibeam Pulsar Survey. Although the objects rotate every few seconds, just like normal (albeit rather slow) pulsars, they only give off a brief radio blip every few minutes or even every few hours, and no one knows why. "That makes them very hard to find," says Lyne, who announced the discovery at a meeting in honor of Ed van den Heuvel here last week. Lyne's team is calling the new class of pulsars Repeating Radio Transients.
The discovery, says Michiel van der Klis of the University of Amsterdam, may shed light on the number and distribution of neutron stars in the Milky Way, and thus on the ultimate fate of massive stars. The true nature of these pulsars is a mystery, but Chryssa Kouveliotou of NASA's Marshall Space Flight Center in Huntsville, Alabama, thinks they may be akin to magnetars--neutron stars with extremely strong magnetic fields. Just like magnetars, the silent pulsars rotate rather slowly, and they seem to have strong magnetic fields too.