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Universality of dual mass-scaled fundamental modes in two-fluid neutron stars with mirror dark matter

Hajime Sotani1,2,3,4,* and Ankit Kumar5,6,†

  • *Contact author: sotani@yukawa.kyoto-u.ac.jp
  • †Contact author: ankitlatiyan25@gmail.com
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Phys. Rev. D 114, 043064 – Published 24 August, 2026

DOI: https://doi.org/10.1103/7tjf-5ggb

Abstract

Universal relations provide a particularly useful way to extract physical information from neutron star observables in the presence of various uncertainties by reducing the dependence on uncertain model parameters and microphysical inputs. In this study, we examine the oscillation frequencies of mirror dark matter admixed neutron stars using a two-fluid description, where the outer and inner fluids give rise to two distinct fundamental frequencies. We confirm that the universal relation between the mass-scaled fundamental frequency of the outer-fluid-led mode and the stellar compactness, established previously for self-interacting dark matter admixed neutron stars, also holds in the mirror dark matter scenario. However, this universal relation becomes less robust when metric perturbations are included, compared with the corresponding results in the Cowling approximation. We further find that the inner-fluid-led fundamental frequency can also be expressed as a compactness-dependent relation that is largely independent of the normal matter equation of state, provided that the dark matter mass fraction is fixed. These results suggest that the simultaneous detection of the two fundamental frequencies could provide a way to constrain the dark matter mass fraction, even when the equation of state of normal matter remains uncertain. Finally, we find that the Cowling approximation estimates the fundamental frequency associated with the outer fluid with an accuracy comparable to that found for standard neutron stars without dark matter, while it performs even better for the frequency associated with the inner fluid.

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Information

Phys. Rev. D 114, 043064– Published 24 August, 2026
  • Received 16 June 2026
  • Accepted 29 July 2026
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