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January 05, 2007
ORNL Team Discovers New Way to Spin Up Pulsars
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In 1967, a Cambridge University graduate student poring over data from a newly constructed radio telescope noticed something very odd--a radio signal blinking regularly from a far corner of the sky.
There was no natural source known to produce such a signal. Early explanations varied, with possible sources ranging from interference by television signals to communications from extraterrestrial beings. The facetious name given at first for this odd, regular blip, in fact, was LGM-1, with the LGM standing for "little green men."
The signal, it turns out, did not come from space aliens; it came from a pulsar, the spinning remnant of a great stellar explosion. Pulsars are the enormously massive--smaller than the moon, heavier than the sun--spinning leftovers from core-collapse supernovas, the cataclysms that provided most of the elements on earth and made our own lives possible.
The leftover core is known as a neutron star, and a neutron star that spins is known as a pulsar. A pulsar appears to blink because radiation shoots out of its magnetic poles, which, as with the earth, can be tilted a little from its axis of spin. As a result, the pulsar behaves like a stellar lighthouse, pointing at an observer once with each rotation.
Nearly four decades later, a team of scientists using Oak Ridge National Laboratory (ORNL) supercomputers has discovered the first plausible explanation for a pulsar's spin that fits the observations made by astronomers. Their surprising results show that the spin of a pulsar is not just a continuation from the massive star that preceded it; in fact, the pulsar spin can be in the opposite direction.
Anthony Mezzacappa of ORNL and John Blondin of North Carolina State University explain their results in the January 4, 2007, issue of the journal Nature. According to three-dimensional simulations they performed at the Leadership Computing Facility, located at ORNL, the spin of a pulsar is determined not by the spin of the original star, but by the shock wave created when the star's massive iron core collapses.
That shock wave is inherently unstable, a discovery the team made in 2002, and eventually becomes cigar-shaped instead of spherical. The instability creates two rotating flows--one in one direction directly below the shock wave and another, inner flow, that travels in the opposite direction and spins up the core.
"The stuff that's falling in toward the center, if it hits this shock wave that is not a sphere any more but a cigar-shaped surface, will be deflected," Mezzacappa explained. "When you do this in 3-D, you find that you wind up with not only one flow, but two counterrotating flows."
The discovery comes at an opportune time, because astronomers did not have a workable explanation for how the pulsar gets its spin. The assumption to this point has been that the spin of the leftover collapsed core comes from the spin of the original star. Being much smaller, the pulsar would then spin much faster than the original star, just as a figure skater spins faster by pulling his or her arms in.
The problem with that approach is that it would explain only the fastest observed pulsars. The ORNL team, on the other hand, predicts spin periods that are in the observed range between 15 and 300 milliseconds.
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