By Paul Rincon

Science reporter, BBC News, Seattle



Crab Nebula: 6,500 light-years away in the constellation of Taurus

Neutron stars are the dense remnants of massive stars that have met their demise in supernova explosions.

Some are called pulsars, because they emit lighthouse-like beams of radio waves that sweep across the Earth.

The profiles of this star's pulses suggest the magnetic field that drives its emissions is highly unusual.

If the findings were confirmed, the neutron star would be the first cosmic object found to have four poles, said Tim Hankins, from the Arecibo Observatory in Puerto Rico and lead investigator on the study.

Megapulse

Ordinarily in a two-pole spinning neutron star, depending on the line of view from Earth, either one or two pulses will be detected.

And the Crab Nebula pulsar is one of those for which two pulses can be identified.

One of these beams is called the main pulse. Occasionally, this produces enormously strong pulses called "megapulses". The other beam is called the interpulse.

The profiles of pulses from the north and south poles of a neutron star should be identical, said Professor Hankins.

Instead, the team found that the two pulses were dramatically different in their profiles, the first time this has been seen in a pulsar.

"We think we've got a much more complicated magnetic field than the simple dipole model," he added.

"What we think is that there is another pole, possibly with a partner, that is influencing and distorting the magnetic field," the Arecibo Observatory astronomer said.

Frozen pole

Magnetic poles always come in pairs, so the fourth is a distinct likelihood.

This neutron star was born in a supernova explosion that was observed on Earth on 4 July, AD 1054.

Professor Hankins said the magnetic poles were essentially "frozen" into the remnant after the supernova explosion.

The cosmic explosion may have been very asymmetric: "Any models you see of supernova explosions are incredibly convoluted. It just doesn't go down as sphere and rebound as a nice sphere.

"The magnetic pole is frozen in so it gets all mixed up as well."

Paul.Rincon-INTERNET@bbc.co.uk