Axion condensates in neutron stars and radial oscillation modes

Light QCD axions, introduced to solve the strong CP problem, may form condensates inside neutron stars, giving rise to a novel ground state of dense matter. We investigate how such axion condensates modify the equilibrium structure and radial oscillation spectrum of neutron stars. Using a realistic neutron star model with the BSk26 equation of state, and solving the coupled Tolman-Oppenheimer-Volkoff and Klein-Gordon equations together with a linear perturbation analysis, we find two distinct families of quasinormal modes: weakly damped fluid-dominated oscillations and highly damped axion modes. The coupling between the fluid and the axion field introduces axion-induced damping of radial oscillations, with decay timescales of order seconds for kHz axion masses. Modes with frequencies above the axion mass are strongly damped, while those below remain unaffected. Although neutron star radial oscillations are difficult to observe, our results suggest that extensions of this work can turn neutron star seismology into a novel probe of the axion properties.

💡 Research Summary

This paper investigates the consequences of light QCD axions forming a condensate inside neutron stars, thereby creating a novel ground state (NGS) of dense nuclear matter. The authors begin by reviewing the axion’s role in solving the strong CP problem and its relevance as a dark‑matter candidate, then introduce a small dimensionless parameter ϵ that suppresses the axion mass relative to the standard QCD axion. In dense matter, the nucleon scalar density n_s can exceed a critical value n_c, causing the effective axion potential to flip sign and driving the axion field from a = 0 to a ≈ ±πf_a. This sourcing of the axion reduces the effective nucleon mass by ∼30 MeV and adds a term –n_bσ_N