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Astronomers in a Spin About Mystery Pulsar

16 May 2008 (All day)
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Bill Saxton, NRAO/AUI/NSF

Strange pair.
Pulsar J1903+0327 appears to be revolving around a sunlike companion star in an elongated orbit.

Imagine a star more massive than the sun yet hardly larger than a small city. Now twirl it around as fast as a kitchen blender spins. Such millisecond pulsars are fairly common in the Milky Way, but astronomers have found one that stands out. Pulsar J1903+0327, some 20,000 light-years from Earth, appears to be orbiting the wrong kind of star in the wrong way. Figuring out what's up with this outlier may shed light on how the most extreme objects in the universe form.

Pulsars are the incredibly dense remains of massive stars that have died in catastrophic supernova explosions. If the star were paired with another star, the pulsar may suck in matter from its companion, causing it to twirl hundreds of times per second. This occurs when its companion becomes a big, cool star known as a red giant, before ending its life as a small and compact white dwarf star. Due to their mutual gravity, the millisecond pulsar and white dwarf enter into a perfectly round orbit around each other.

That's why J1903+0327 is such an odd duck. Discovered in 2005 by the 300-meter radio telescope in Arecibo, Puerto Rico, the pulsar rotates on its axis 465 times every second, pegging it as a sure-fire millisecond pulsar--one of the fastest known, in fact. But a large international team of astronomers led by David Champion of the Australia Telescope National Facility in Epping, New South Wales, reports online this week in Science that the pulsar is in an eccentric 95-day orbit. Not only that, but it's apparently circling a star resembling our own sun.

In the paper, the team says the most likely explanation for the strange find is the presence of a third, as-yet-undiscovered star in the system. Instead of orbiting the sunlike star, the pulsar orbits the third star, the team speculates, while the sunlike star circles the pair at a much larger distance, disturbing what should be a perfectly circular orbit.

"It's a very exciting and unprecedented result," says Jonathan "Josh" Grindlay of Harvard University. But he has a different theory. Grindlay thinks the mystery millisecond pulsar was born about a billion years ago in a globular cluster, a dense, spherical swarm of hundreds of thousands of stars. Originally, he says, the pulsar would have orbited a white dwarf, but in a close encounter with a normal star--which happens frequently in dense globular clusters--the pulsar would have swapped partners, ended up in an elongated orbit, and gotten ejected from the cluster. "Follow-up observations will hopefully solve the riddle once and for all," Grindlay says.

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