Neutron¹ stars, the extraordinarily² dense³ stellar bodies created when massive stars collapse⁴, are known to host the strongest magnetic fields in the universe -- as much as a billion times more powerful than any human-made electromagnet. But some neutron stars are much more strongly magnetized than others, and this disparity has long puzzled astrophysicists（天体物理学家）. Now, a study by McGill University physicists⁵ Konstantinos Gourgouliatos and Andrew Cumming sheds new light on the expected geometry of the magnetic field in neutron stars. The findings, published online April 29 in Physical Review Letters, could help scientists measure the mass and radius⁶ of these unusual stellar bodies, and thereby⁷ gain insights into the physics of matter at extreme densities⁸.

 

Some previous theoretical studies have suggested that the magnetic field of a neutron star should break into smaller loops and dissipate（驱散，浪费） as the star ages -- a phenomenon known as "turbulent cascade⁹." Yet, there are several "middle-aged¹⁰" neutron stars (roughly one million to a few million years old) that are known to have relatively¹¹ strong magnetic fields, leaving scientists at a loss to reconcile（调停） the theoretical models with actual observations.

 

To better understand how the magnetic field changes as a neutron star ages, Gourgouliatos and Cumming ran a series of computer simulations. These showed the magnetic field evolving rapidly at first, in line with previous predictions. But then the evolution took a surprising turn: in all the simulations, no matter what the magnetic field looked like when the neutron star was born, the field took on a particular structure and its evolution dramatically slowed.

 

"A cascade in a magnetic field is akin¹² 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," Cumming explains. "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 random¹³ directions. Instead, we find in these new simulations that the magnetic field actually remains¹⁴ quite simple in structure -- as if the cream refused to mix into the coffee -- and you could, indeed, use a compass to navigate¹⁵ around on the surface of the star."

 

The McGill researchers call this final magnetic-field configuration¹⁶ the "Hall attractor" state, after the so-called Hall effect, which is thought by astrophysicists to drive magnetic field evolution in neutron-star crusts. "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 turbulence¹⁷, can actually lead to an attractor state with a stable magnetic-field structure," Gourgouliatos says.

 

The research was supported by the Centre de Recherche¹⁸ en Astrophysique du Québec and the Natural Sciences and Engineering Research Council of Canada.

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