Supramassive neutron stars (SMNSs) are uniformly rotating equilibrium configurations with masses exceeding the maximum mass of nonrotating solutions (Tolman–Oppenheimer–Volkoff) for a given equation of state. Their stability is well understood in absence of magnetic fields but the “simplest” magnetic field configurations are instead known to be unstable. Modeling pulsars and magnetars therefore requires a better understanding of the stability of SMNS with extreme magnetic fields.
In 2610.28609 we investigate the dynamical stability of strongly magnetized supramassive neutron stars close to the mass-shedding limit using fully general-relativistic magnetohydrodynamic simulations in three spatial dimensions. Equilibrium configurations are constructed using a microphysical equation of state and considering poloidal magnetic fields up to maximum strengths ∼1e17G. The initially unstable magnetic fields reorganize into mixed poloidal–toroidal configurations on Alfvén timescales shorter than the stellar rotation period. The subsequent dynamical evolution is governed by timescales set by the Lehnert number. For stars with masses sufficiently lower than the maximum mass, magnetic stresses redistribute angular momentum and drive the development of differential rotation, leading to a new quasi-stationary configuration that remains dynamically stable over several rotation periods. Magnetically-driven winds develop from the outer layers around the equatorial plane and remain baryon-matter dominated with magnetization ≲0.2. Over the simulated timescale, winds carry a total rest mass of order ∼10−3Mo at outflow rates of up to a few Mo/s. For stars close to (or beyond) the maximum-mass turning point along the stellar equilibrium sequence, strong magnetic fields trigger gravitational collapse by redistributing angular momentum away from the stellar core. The resulting black hole remnants are characterized by angular velocities up to ∼49% larger and angular momentum ≲8% smaller than those of the initial equilibrium stars. Our results show that extreme magnetic-field instabilities can significantly alter the structure of supramassive neutron stars, with possible implications for magnetars and long-lived binary neutron-star merger remnants.
