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Nonradial pulsations of gravitationally coupled two-fluid neutron stars in general relativity

Ankit Kumar1,2,*, Daniel A. Caballero3,†, Hajime Sotani4,5,6,7,‡, and Nicolás Yunes3,§

  • *Contact author: ankitlatiyan25@gmail.com, ankitkumar2@iisc.ac.in
  • †Contact author: dac8@illinois.edu
  • ‡Contact author: sotani@yukawa.kyoto-u.ac.jp
  • §Contact author: nyunes@illinois.edu
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Phys. Rev. D 114, 043051 – Published 20 August, 2026

DOI: https://doi.org/10.1103/642h-xhjf

Abstract

Nonradial oscillations of dark matter admixed neutron stars offer a powerful probe of stellar structure and relativistic gravity, but when the ordinary and dark matter components are modeled as dynamically distinct fluids, their consistent description requires a two-fluid treatment that retains the dynamical response of both components and their coupling to spacetime. The system considered here corresponds to the noninteracting limit of the established general relativistic formalism for polar perturbations of two-fluid stars, but is formulated directly for independently conserved perfect-fluid components that interact only through gravity, without entrainment or direct microphysical coupling. Within this formulation, we present the coupled linear perturbation equations for the spacetime metric and both fluid components, together with the central regularity conditions, the boundary conditions at the individual fluid surfaces, and the matching to the exterior vacuum spacetime required to determine the polar mode spectrum. We implement this system numerically for mirror dark matter admixed neutron stars, describing both components with the QHC21-BT equation of state, and compute representative polar mode spectra. The resulting spectra contain distinct fundamental (f) and pressure (p) mode branches with dominant inner- or outer-fluid character, which we identify from the associated eigenfunctions and their radial node structure. We find that the additional gravitationally coupled fluid shifts the ordinary-matter-led f-mode branch and that the equation-of-state-insensitive universal relation between the mass-scaled f-mode frequency and compactness known for single-fluid neutron stars is not preserved in the same form. These results provide a fully general relativistic characterization of the polar mode spectrum of mirror dark matter admixed neutron stars and illustrate the asteroseismic signatures introduced by an additional gravitationally coupled fluid component.

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Phys. Rev. D 114, 043051– Published 20 August, 2026
  • Received 5 May 2026
  • Accepted 4 August 2026
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