Astronomers may have to go back to the drawing board after the discovery of an unusual pulsar, which doesn't appear to be slowing down.
Reporting in the journal Science, Dr Silvia Zane and colleagues from the University College London, say the pulsar, named SGR-0418+5729, may help unlock the internal secrets of these once giant stars.
Pulsars are rapidly spinning neutron stars formed from the collapsed core of a star 10 to 50 times more massive than the Sun, that exploded as a supernova after running out of fuel.
While the external layers of the star are blown into space by the supernova event, its nucleus collapses under its own gravity into a sphere only 30 kilometres in diameter. Protons and electrons within the collapsed star are crushed to form neutrons, hence the name.
The most powerful neutron stars, called magnetic stars or magnetars, generate magnetic fields a thousand times stronger than normal neutron stars. They also emit massive flares of gamma and x-rays.
Current theory suggests a magnetar's internal magnetic field is stronger than its surface field, causing the crust to fracture, allowing the magnetic field to propagate outwards.
Only a half dozen or so magnetars have ever been identified.
Stranger and stranger
On 5 June 2009 the Fermi Gamma-Ray Observatory detected two magnetar-like bursts coming from SGR-0418.
Using the XMM-Newton, Chandra and Swift X-ray Observatories, Zane and colleagues examined the pulsar and found it has a large internal magnetic field, but a weak magnetic field on its surface.
"It's the very first time this has been observed and the discovery poses the question of where the powering mechanism is in this case," says Zane.
As pulsars rotate, winds of high energy particles carry energy away from the star causing the rotation rate to gradually decrease. But according to Zane, SGR-0418 is showing no evidence of slowing down.
"[It] raises questions about how many other normal looking pulsars out there can at some point wake up and manifest themselves as a flaring source," he says.
Lead researcher, Dr Nanda Rea of Institut de Ciencies de l'Espai in Barcelona, says astronomers may have to rethink how pulsars and magnetars function.
"If further observations by Chandra and other satellites push the surface magnetic field limit lower, then theorists may have to dig deeper for an explanation of this enigmatic object."