Technology Review – pulsars are rotating neutron stars emitting radiation from their magnetic poles. They appear to pulse because the magnetic axis is not aligned with the axis of rotation, so the pole comes in and out of view as the neutron star rotates. The conventional view is that their magnetic field arises from the movement of charged particles as they rotate. These charged particles ought to behave like a superfluid and so should end up becoming aligned with the axis of rotation. These kinds of superfluid currents are likely to be highly unstable, generating wobbles in the magnetic field. But pulsars are well known for being amazingly stable. How can this be?
Another problem is how pulsars end up with magnetic fields that are so strong. The conventional view is that the process of collapse during a supernova somehow concentrates the original star’s field. However, a star loses much of its material when it explodes as a supernova and this presumably carries away much of its magnetic field too. But some pulsars have fields as high as one trillion Tesla, far more than can be explained by this process.
Now researchers suggest that neutron stars are giant permanent magnets. They point out that there is another way for magnetic fields to form, other than the movement of charged particles. This other process is by the alignment of the magnetic fields of the body’s components, which is how ferromagnets form.
Their suggestion is that when a neutron star forms, the neutron magnetic moments become aligned because this is the lowest energy configuration of the nuclear forces between them. When this alignment takes place, a powerful magnetic field effectively becomes frozen in place.
We argue that pulsars may be spin-polarized neutron stars, i.e. cosmic permanent magnets. This would simply explain several observational facts about pulsars, including the ‘beacon effect’ itself i.e. the static/stable misalignment of rotational and magnetic axes, the extreme temporal stability of the pulses and the existence of an upper limit for the magnetic field strength – coinciding with the one observed in “magnetars”. Although our model admittedly is speculative, this latter fact seems to us unlikely to be pure coincidence.
The theory is testable too. It predicts that neutron stars cannot have magnetic fields greater than 10^12 Tesla. So the discovery of a neutron star with a stronger field would immediately scupper it.
But the idea also raises some questions of its own. Not least of these is whether it is even possible for neutron magnetic moments to become aligned in the way Hansson and Ponga suggest. The Pauli exclusion principle would, at first sight, seem to exclude the possibility of neutrons being aligned in this way.