Original title: Interior Magnetic Fields in Magnetars and Radio Pulsars
Raj Kishor Joshi, Brynmor Haskell, William Cook, Sebastiano Bernuzzi · September 2026
Neutron stars have powerful magnetic fields, but scientists cannot directly observe the fields hidden inside them. To solve this, researchers ran 3D computer simulations that model magnetic fluids in curved spacetime across different spin speeds and field strengths. They found that a star's spin speed determines its internal magnetic shape. When a star spins rapidly, its spin period is shorter than the time magnetic waves take to cross it. This fast rotation winds up the field, amplifying a strong, doughnut-shaped toroidal field before instabilities can destroy it, creating a stable interior. When magnetic forces dominate instead, unstable field loops decay quickly, leaving behind weak toroidal fields. This explains why fast-spinning radio pulsars build strong toroidal fields inside, while magnetarsNeutron stars with extremely strong magnetic fields that cause violent outbursts. end up with mostly pole-to-pole poloidal fields.
Imagine a dancer spinning on ice. If they start fast and keep their arms in, they spin smoothly. If they try to spin with arms outstretched and start slow, they quickly wobble and fall. In this paper, the fast spinner develops a stable internal twist (toroidal field), while the slow spinner's field collapses to a simple bar magnet shape (poloidal).
Neutron stars are cosmic laboratories, but their interiors are invisible. This work shows that a star's spin history determines whether it becomes a magnetar or a radio pulsar. That explains why some neutron stars flare violently while others pulse steadily, connecting decades of scattered observations to a single physical cause.
The abstract does not specify direct observational tests or exact timescales for the evolutionary regimes. The simulations cover a range of rotation rates and field strengths but may not include all possible neutron star scenarios. The study does not address the formation process of the initial magnetic fields or how the internal configurations relate to specific emission mechanisms.
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Interior Magnetic Fields in Magnetars and Radio Pulsars — Raj Kishor Joshi, Brynmor Haskell, William Cook et al.
Abstract licence: CC0
Explanation generated by Synthesis from the abstract — it is not the original paper. This is a preprint, so it hasn't necessarily been peer reviewed.