Towards constraining axions with polarimetric observations of the isolated neutron star RX J1856.5-3754

TL;DR

This study explores how photon-axion mixing affects the polarization and spectra of isolated neutron stars, aiming to use polarimetric data to constrain axion properties.

Contribution

It provides a detailed analysis of axion-photon mixing effects on neutron star polarization, using realistic surface emission models and focusing on RX J1856.5-3754.

Findings

Photon-axion mixing limits the configurations reproducing observed polarization.

Mixing effects are negligible for atmospheric surface models.

Upcoming X-ray polarimetry can improve constraints on axion parameters.

Abstract

Photon-axion mixing can create observable signatures in thermal spectra of isolated, cooling neutron stars. Their shape depends on the polarization properties of the radiation, which, in turn, are determined by the structure of stellar outermost layers. Here we investigate the effect of mixing on the spectrum and polarimetric observables, polarization fraction and polarization angle, using realistic models of surface emission. We focus on RX J1856.5-3754, the only source among the X-ray dim isolated neutron stars for which polarimetric measurements in the optical band were performed. Our results show that in the case of a condensed surface in both fixed and free-ion limits, the mixing can significantly limit the geometric configurations which reproduce the observed linear polarization fraction of 16.43%. In the case of an atmosphere, the mixing does not create any noticeable signatures. Complementing our approach with the data from upcoming soft X-ray polarimetry missions will allow to obtain constraints on $g_{\gamma a} \sim 10^{-11}$ GeV$^{-1}$ and $m_a \lesssim 10^{-6}$ eV, improving the present experimental and astrophysical limits.