Axisymmetric toroidal modes of general relativistic magnetized neutron star models

Abstract

We calculate axisymmetric toroidal modes of magnetized neutron stars with a solid crust in the general relativistic Cowling approximation. We assume that the interior of the star is threaded by a poloidal magnetic field, which is continuous at the surface with an outside dipole field. We examine the cases of the field strength $B_{{\rm{S}}}\sim10^{16}$ G at the surface. Since separation of variables is not possible for the oscillations of magnetized stars, we employ finite series expansions for the perturbations using spherical harmonic functions. We find discrete normal toroidal modes of odd parity, but no toroidal modes of even parity are found. The frequencies of the toroidal modes form distinct mode sequences and the frequency in a given mode sequence gradually decreases as the number of radial nodes of the eigenfunction increases. From the frequency spectra computed for neutron stars of different masses, we find that the frequency is almost exactly proportional to $B_{{\rm{S}}}$ and is well represented by a linear function of $R/M$ for a given $B_{{\rm{S}}}$, where $M$ and $R$ are the mass and radius of the star. The toroidal mode frequencies for $B_{{\rm{S}}}\sim 10^{15}$ G are in the frequency range of the quasi-periodic oscillations (QPOs) detected in the soft-gamma-ray repeaters, but we find that the toroidal normal modes cannot explain all the detected QPO frequencies.