Spacetime quasinormal mode oscillations of anisotropic neutron stars
Phys. Rev. D 114, 044052 – Published 17 August, 2026
DOI: https://doi.org/10.1103/g1mt-4d4d
Abstract
Neutron star asteroseismology offers a unique opportunity to probe nuclear physics through stellar oscillations. Although the pressure inside of a neutron star is typically assumed to be isotropic, pressure anisotropy can arise from various physical mechanisms, including elasticity, viscosity, and magnetic fields. Previous studies of nonradial stellar quasinormal mode oscillations with anisotropic pressure have focused primarily on fluid modes. In this paper, we compute, for the first time, spacetime oscillation modes (so-called -modes) of anisotropic neutron stars. Using a perturbative framework for stellar oscillations with pressure anisotropy, developed previously by some of the authors, together with a phenomenological anisotropy model, we find that both the real and imaginary parts of the -mode frequencies decrease as the tangential pressure becomes dominant over the radial pressure. Although we do not find any unstable -modes within the physically viable parameter space, unstable -modes appear in an unphysical branch of solutions when the tangential pressure strongly dominates the radial one. We also find that the relation between the real part of the -mode frequency and the stellar compactness is quasiuniversal with respect to variations in the equation of state and may become insensitive to the degree of pressure anisotropy as well for some anisotropy models. In contrast, the relation between the imaginary part of the -mode frequency and the stellar compactness depends on the degree of anisotropy but remains equation-of-state universal when the anisotropy is fixed. Finally, we discuss potential mode crossings and the validity of certain approximations that have been shown to work well for -mode calculations in the isotropic case.
