Constraints on Axion-Photon Mixing from Fast Radio Burst Dispersion Measures

Abstract

Fast radio bursts (FRBs) offer a powerful probe of the ionized Universe through their dispersion measures (DM). While a significant fraction of the DM arises from the intergalactic medium (IGM), the contributions from the host galaxy and the immediate environment of the source remain uncertain, and the physical origin of FRBs is still under active investigation. In this work, we investigated the possibility that FRBs originate from high-magnetic-field neutron stars (NS), whose magnetospheres can facilitate axion-photon mixing. Such mixing can modify photon propagation and induce an effective contribution to the observed dispersion. Using a sample of localized FRBs with measured redshifts, we perform a Bayesian Markov Chain Monte Carlo (MCMC) analysis to constrain the axion mass $m_a$ and axion-photon coupling $g_{aγγ}$. Within a parametric cosmological framework, we obtain $m_a = 1.16^{+4.40}_{-1.08}\,μ{\rm eV}$ and $g_{aγγ} = (1.76^{+6.69}_{-1.64})\times10^{-16}\,{\rm GeV}^{-1}$, together with a physically consistent intergalactic baryon fraction $f_{\rm IGM} = 0.837^{+0.053}_{-0.056}$. We further tested the robustness of our bounds against cosmological modeling assumptions by employing a non-parametric Gaussian Process reconstruction (GPR) of the DM-$z$ relation, which gives statistically consistent results.

Figures (6)

• Figure 1: Schematic illustration of the NS magnetosphere and the geometry relevant for axion-photon mixing. The dashed and solid circles indicate the conversion radius $r_{\rm c}$ and the light-cylinder radius $R_{\rm LC}$ respectively. This schematic figure is inspired from Berghaus_2025.
• Figure 2: DM as a function of $z$ for the FRB sample. The solid line shows the Gaussian Process posterior mean obtained with a Matérn kernel, and the shaded region indicates the $1\sigma$ confidence band. DM values used in the reconstruction are listed in Appendix A.
• Figure 3: Marginalized posterior distributions for the model parameters $m_a \ (\mu{\rm eV})$, $g_{a\gamma\gamma} \ ({\rm GeV}^{-1})$, $\mathrm{DM}_{\rm local}$, and $f_{\rm IGM}$ inferred from the MCMC analysis of FRBs. The 1D and 2D contours correspond to the 68% and 95% credible regions.
• Figure 4: Observed DM of 125FRBs as a function of $z$ . The red curve shows the standard Macquart relation with a $1\sigma$ scatter band, adopting fixed foreground contributions commonly used in the literature. The green curve indicates the posterior mean prediction of the axion model obtained in this work, with the shaded region representing the corresponding $1\sigma$ uncertainty.
• Figure 5: The schematic bounds approximately shows the summary of existing constraints on the axion-photon coupling compared with the FRB-derived limits obtained in this work. All other bounds we obtain from the AxionLimits compilation AxionLimits.