Saturday, November 2, 2013

Crab Nebula's Strange Pulsar Heart Slowly Going Off-Kilter

A composite image of the Crab Nebula showing the X-ray (blue), and optical (red) images superimposed. 

The size of the X-ray image is smaller because the higher energy X-ray emitting electrons radiate away their energy more quickly than the lower energy optically emitting electrons as they move.

Credit: NASA/HST/ASU/J. Hester et al. X-Ray: NASA/CXC/ASU/J. Hester

For the first time, astronomers have tracked the evolution of a pulsar's magnetic field over time, watching as it slowly tilts toward the dead star's equator.

The new observations of the pulsar, located in the Crab Nebula, could offer clues to the long-standing problem of what slows pulsars' rotation.

Andrew Lyne
"Most pulsars are millions or tens of millions of years old," said Andrew Lyne, emeritus professor of physics at the University of Manchester in the U.K., who led the study, which appears in the Nov. 1 issue of the journal Science.

"So we don't expect to see significant changes. But we have been looking at this for a substantial portion of its lifetime, some 40 out of 1,000 years."

The supernova that birthed the pulsar in the Crab Nebula occurred in A.D. 1054. Chinese and Arab astronomers both noted it.

"It's a result we've waited 30 years for," said Vasily Beskin, an astrophysicist at the Russian Academy of Sciences.

Beskin, who was not involved in the study, and his colleagues predicted that pulsar magnetic fields would move to their equators in the 1980s.

The new data also gave other insights. "Normally, magnetic fields don't move through superconductors," Lyne said. "This magnetic field is moving, which suggests the superconductor in the neutron star is not perfect."

It's not likely that astronomers will run across another like the Crab pulsar, because to see one at all, the radio beam has to sweep across the Earth, and the odds of one being in precisely the right orientation are small.

On top of that, the supernova that made the pulsar would have to be less than a few thousand years old, scientists say.

There are several supernovas of the correct age, but they aren't all the right type to produce pulsars, and even if they were, they aren't pointed the right way.

It still isn't completely clear why pulsars' magnetic fields look as they do. "I wouldn't class it as being a simple problem," Lyne said. "We're trying to understand why it should evolve in this way."

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