magnetar surrounded by magnetic field lines
May 7, 2014

McGill Physicists Shed New Light On The Expected Geometry Of Neutron Star Magnetic Fields

John P. Millis, Ph.D. for - Your Universe Online

Neutron stars – also called pulsars for the way they send pulses of light across the Universe due to their high spin rates – are some of the most dynamic objects in the Universe, characterized by incredibly compact dimensions, focused beams of radiation, and incredible surface gravity. Additionally, these rapid rotators possess the strongest magnetic fields known to man – billions of times more powerful than anything ever created in a laboratory.

The presence of such strong fields are thought to be the primary mechanism responsible for production of the electromagnetic radiation observed in nearly all neutron star systems. Recent studies are even finding unique ways of measuring the near-surface fields to analyze how the radiation is generated across the radio band.

But as amazing as neutron star magnetic fields are in general, some are unusually strong – including a subclass of pulsars known as magnetars, which sustain magnetic fields some 100 times more powerful than most – even by pulsar standards. Understanding these outliers has proven difficult, however.

In essence, astronomers have usually modeled neutron star magnetic fields as breaking apart into smaller loops that then dissipate as the pulsar ages – a process known in theoretical physics circles as a turbulent cascade. However, such models can’t explain pulsars which are around a few million years old and still maintain strong magnetic fields.

In order to attempt to reconcile the theoretical models with observational data, McGill University physicists took to modeling the evolution of the magnetic fields within these systems.

Initially the fields rapidly evolved, just as previous models predicted. However, in every system that the team modeled, the magnetic field structure took to a particular form common to all pulsars, regardless of what the fields look like when the pulsar was born. And, at this point, the evolution slowed, meaning the neutron star was able to maintain its field strength much longer than previously thought possible.

“A cascade in a magnetic field is akin to what happens when you add cream to your coffee and stir it: the cream rapidly gets broken up into pieces and mixes into the coffee,” explains Andrew Cumming from McGill University and co-author of the study in a recent statement. “The original prediction was that neutron star crusts would do the same to their magnetic fields; so if you could walk around on the surface with a compass trying to walk towards magnetic north, you would end up walking around in random directions. Instead, we find in these new simulations that the magnetic field actually remains quite simple in structure – as if the cream refused to mix into the coffee – and you could, indeed, use a compass to navigate around on the surface of the star.”

The team dubbed this result the “Hall attractor” state, after the so-called Hall effect, which is believed to drive magnetic field evolution in neutron stars. According to Konstantinos Gourgouliatos, the second McGill physicist involved in the study, “This result is also significant because it shows that the Hall effect, a phenomenon first discovered in terrestrial materials and which is thought to help weaken a magnetic field through turbulence, can actually lead to an attractor state with a stable magnetic-field structure.”

The results of this work, “Hall Attractor in Axially Symmetric Magnetic Fields in Neutron Star Crusts”, by Konstantinos N. Gourgouliatos and Andrew Cumming, were published in the journal Physical Review Letters on April 29, 2014.