Axion-photon conversion in transient compact stars: Systematics, constraints, and opportunities

Abstract

We study magnetic conversion of ultra-relativistic axion-like particles (ALPs) into photons in compact-star environments, focusing on the hot, transient conditions of core-collapse supernova (SN) remnants and neutron-star mergers (NSMs). We address previously overlooked uncertainties, particularly the suppression caused by ejected matter near the stellar surface, a region crucial to the conversion process. We derive analytical expressions for the transition rate; they reveal the influence of key parameters and their uncertainties. We update constraints using historical gamma-ray data from SN~1987A and find $g_{aγ}<5\times10^{-12}~{\rm GeV}^{-1}$ for $m_a\lesssim10^{-9}$ eV. We also forecast sensitivities for a future Galactic SN and for NSMs, assuming observations with Fermi-LAT or similar gamma-ray instruments. We distinguish ALPs -- defined as coupling only to photons and produced via Primakoff scattering -- from axions, which also couple to nucleons and emerge through nuclear bremsstrahlung. We omit pionic axion production due to its large uncertainties and inconsistencies, though it could contribute comparably to bremsstrahlung under optimistic assumptions. For the compact sources, we adopt time-averaged one-zone models, guided by numerical simulations, to enable clear and reproducible parametric studies.

Figures (12)

• Figure 1: Axion-photon conversion sites in the Hillas plane. Lines of constant conversion probability (for massless axions) are shown in blue, while the contours denoting the mass at which the axion conversion is suppressed are shown in gray. We focus on energetic axions produced in hot, compact sources, and correspondingly show the parameters of neutron star mergers (NSMs) 1992ApJ...392L...9DKiuchi:2014hjaKiuchi:2015sgaGiacomazzo:2014qbaMetzger:2018qflKiuchi:2023obeGutierrez:2025gkx, Type Ibc stripped-envelope SNe Candon:2025sdm
• Figure 2: Primakoff and bremsstrahlung flux spectra of several SN models. The spectrum obtained after integrating Eqs. \ref{['eq:Primakoff-emission-rate']} and \ref{['eq:emission_rate_bremsstrahlung']} over the cold (SFHo-18.8) and hot (SFHo-20.0) Garching models are shown in green. The best-fit Gamma distribution with the parameters given in Table \ref{['tab:SN-models']} are shown in orange. In purple we show the emission considering the one-zone models with the parameters given in Table \ref{['tab:one_zone']}. For bremsstrahlung (cold model), the numerical fit given in Ref. Lella:2024hfk is shown in gray. The vertical lines bracket the energy range covered by SMM.
• Figure 3: Primakoff and bremsstrahlung flux spectra emitted by the NSM remnant. We show the spectra obtained after integrating Eqs. \ref{['eq:Primakoff-emission-rate']} and \ref{['eq:emission_rate_bremsstrahlung']} over four Garching models listed in Table \ref{['tab:NSM-models']}. In purple we show the emission by the one-zone model with the parameters given in Table \ref{['tab:one_zone']}. The time integration period is 10 ms.
• Figure 4: Numerical solution of the axion-photon mixing system in the SN case, assuming $a(0)=1$ and $A(0)=0$, and for the reference values of axion energy $E$, mass $m_a$, axion-photon coupling $g_{a\gamma}$, surface magnetic field $B_0$, and progenitor radius $R_0$ as shown in the legend. The contributions from magnetic refraction and plasma density are negligible. The asymptotic value of $|A|^2$ matches with its analytical prediction found in Eq. \ref{['eq: SNProbPrediction']}.
• Figure 5: Axion-photon conversion probability as a function of emission time for two scenarios. Left: quasi-massless axion ($m_a=10^{-10}$ eV), where conversion happens in the region dominated by magnetic-induced refraction, and Right: a massive case ($m_a=10^{-2}$ eV), where instead the mass term $\Delta_a$ dominates. The case including ejecta (blue) is compared with their absence (red), using the same choices of axion energy $E$, axion-photon coupling $g_{a\gamma}$, ejecta velocity $V$, magnetic field at the HMNS surface $B_0$, and radius $R_0$. The black dashed line marks the instant when the ejecta reach the radius where most of the conversion takes place.