Pulsar in Crab Nebula Has Four Poles, Astronomers Suggest

John Roach in Seattle, Washington
for National Geographic News
January 9, 2007
A pulsating star, or pulsar, in the Crab Nebula may be the first known celestial body to have more than two poles, astronomers said Monday.

Most pulsars are dipolar, meaning they have two magnetic poles: north and south. The radio waves that pulse from the poles are identical to each other.

"We've got a much more complicated magnetic field [in the Crab Nebula pulsar] than the simple dipole model," Tim Hankins, acting director of the Arecibo Observatory in Puerto Rico, said at press briefing Monday.

Hankins and colleague Jean Eilek from New Mexico Tech in Socorro studied the radio waves coming from the pulsar's poles and found they are dramatically different.

"What we think is going on is that there is another pole which is influencing and distorting the magnetic field," said Hankins, who is also an emeritus professor at New Mexico Tech.

If there is a third pole then there is presumably a fourth as well, Hankins added, since all magnetic fields are dipolar.

Hankins presented the research at the 209th meeting of the American Astronomical Society in Seattle, Washington.

Crab Nebula

Pulsars are dense, rapidly spinning neutron stars that are relics of supernova explosions—the powerful death throes of massive stars.

The Crab Nebula pulsar rotates 30 times a second and emits strong pulses of radiation that range from radio rays to gamma rays.

The pulsar's strong magnetic field concentrates most of its radiation pulses into cones that beam from the poles, like light beamed from a lighthouse.

Observers detect the pulses whenever these emission cones sweep toward Earth.

(Download a video of the Crab Nebula pulsar in motion [requires RealPlayer].)

For some pulsars, including the Crab Nebula's, astronomers can detect a main pulse and an interpulse, or second pulse. The pulse and interpulse are believed to correspond to the north and south poles and typically resemble each other.

But Hankins and Eilek found that in the Crab Nebula pulsar, the main pulse is characterized by extremely short, powerful bursts, whereas the interpulse is broad and smooth.

The main pulse can last just four-tenths of a nanosecond, Hankins said. These so-called nanoshots are believed to be produced by small plasma clouds in the pulsar's atmosphere that are only 5 inches (12 centimeters) wide.

The interpulse, meanwhile, makes radio emissions unlike any that have been detected before from a pulsar.

This irregularity, as well as the difference between the main pulse and interpulse, cannot be explained by existing pulsar models, the researchers noted.

"This is a cool result," said Eilek, who was unable to attend the conference, in a media statement. "It knocks just about every existing theory of pulsar radio emission for a loop."

Paulo Friere is a research associate at the Arecibo Observatory who is familiar with the result but was not part of the team. He said the interpulse's emission "is one of the weirdest things I've seen in my life."

While he is unable to explain the significance emission, he said, "There's definitely something extremely odd happening with the Crab pulsar."

Violent Collapse

According to Hankins, an additional polar element could be causing the unusual emission.

So how could an additional set of poles have formed?

Hankins said that perhaps it occurred during the formation of the pulsar, which is a violent, complicated, and perhaps asymmetrical process.

The supernova that resulted in the Crab Nebula was observed by Chinese and Japanese astronomers in A.D. 1054.

"This is a very violent explosion, and these extra poles could be remnants from when the pulsar was actually formed a thousand years ago," Hankins said.

Further imaging studies of the pulsar's polar regions and recording the full bandwidth of its radiation may help reveal an answer.

"That's going to have to tell us something about the physics behind this emission mechanism," Hankins said.

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