Interior Magnetic Fields in Magnetars and Radio Pulsars
Magnetic fields are fundamental for neutron star physics and play a central role in powering the extreme phenomenology of magnetars, including Soft Gamma Repeaters and Anomalous X-ray Pulsars. However, the structure and stability of their internal magnetic fields remain largely unconstrained, as they cannot be directly probed by electromagnetic observations. Using 3D general-relativistic magnetohydrodynamics simulations across a range of rotation rates and magnetic field strengths, we identify two distinct evolutionary regimes leading towards dynamically stable magnetic configurations. In rapidly rotating stars, the Alfvén crossing timescale exceeds the rotation period, allowing differential winding to amplify a strong toroidal magnetic component before the onset of instabilities, leading to long-lived, stable configurations. In contrast, in magnetically dominated stars, instabilities in the poloidal field drive rapid field decay, leaving only a comparatively weak toroidal component. These results imply that the internal magnetic structure of neutron stars depends sensitively on their rotational state: rotation-dominated stars like radio pulsars develop strong toroidal fields, while magnetars are characterized by predominantly poloidal configurations. Our findings therefore show that a neutron star's rotational history shapes its internal magnetic structure, providing a unifying physical picture that connects the observed diversity of neutron star classes to their hidden field configurations.