Photon-axion mixing in thermal emission of isolated neutron stars

Thermally emitting neutron stars represent a promising environment for probing the properties of axion-like particles. Due to the strong magnetic fields of these sources, surface photons may partially convert into such particles in a large magnetospheric region surrounding the stars, which will result in distinctive signatures in their spectra. However, the interaction depends on the polarization state of the radiation and is rather weak due to the low experimentally allowed values of the coupling constant $g_{\gamma a}$. In this work, we compute the degree of photon-axion transition in the case of 100% O-mode polarization and spectral energy distribution of an isotropic blackbody with uniform surface temperature. The stellar magnetic field is assumed to be dipolar. We show that for the magnetic fields $\sim {10}^{13}$ – 10¹⁴ G, typical for X-ray dim isolated neutron stars, the maximum effect is reached at $g_{\gamma a}=2\times {10}^{-11}{\text{GeV}}^{-1}$: the optical flux is reduced by 30 – 40%, while the high-energy part of the spectrum is not affected. The low-energy decrease exceeds 5% at $g_{\gamma a}\ge 2\times {10}^{-12}{\text{GeV}}^{-1}$ and $m_a\le 2\times {10}^{-6}$ eV, which is below the present experimental and astrophysical limits on axion parameters. To obtain the actual observational constraints, rigorous treatment of the radiative surface layers is required.