Photon-axion mixing in thermal emission of isolated neutron stars

TL;DR

This paper investigates how axion-like particles could affect the spectra of isolated neutron stars through photon conversion in strong magnetic fields, potentially leading to observable spectral signatures.

Contribution

It provides a detailed calculation of photon-axion conversion effects in neutron star environments, considering polarization and magnetic field configurations, and identifies potential spectral signatures.

Findings

Optical flux reduction of 30-40% at certain magnetic fields and coupling constants.

High-energy spectrum remains unaffected by photon-axion conversion.

Low-energy spectral decrease exceeds 5% for specific axion parameters.

Abstract

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 the 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 with the maximum effect reached for the magnetic fields $\sim10^{13}$ - $10^{14}$ G (typical for X-ray dim isolated neutron stars) and $g_{\gamma a} = 2 \times 10^{-11}$ 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 $g_{\gamma a} \geq 2 \times 10^{-12}$ GeV$^{-1}$ and $m_a \leq 2\times 10^{-6}$ 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.