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In the strong magnetic field of a neutron star’s magnetosphere, axions coupled to electromagnetism develop a nonzero probability to convert into photons. Past studies have revealed that the axion-photon conversion can be resonantly enhanced. We recognize that the axion-photon resonance admits two parametrically distinct resonant solutions, which we call the mass-matched resonance and the Euler-Heisenberg assisted resonance. The mass-matched resonance occurs at a point in the magnetosphere where the radially-varying plasma frequency crosses the axion mass ${\omega }_{\mathrm{pl}}\approx m_a$. The Euler-Heisenberg assisted resonance occurs where the axion energy satisfies $\omega \approx (2{\omega }_{\mathrm{pl}}^2/7g_{\gamma \gamma \gamma \gamma }{\overline{B}}^2{)}^{1/2}$. This second resonance is made possible though the strong background magnetic field $\overline{B}$, as well as the nonzero Euler-Heisenberg four-photon self-interaction, which has the coupling $g_{\gamma \gamma \gamma \gamma }=8{\alpha }^2/45m_e^4$. We study the resonant conversion of relativistic axion dark radiation into photons via the Euler-Heisenberg assisted resonance, and we calculate the expected electromagnetic radiation assuming different values for the axion-photon coupling $g_{a\gamma \gamma }$and different amplitudes for the axion flux onto the neutron star ${\mathrm{\Phi }}_a$. We briefly discuss several possible sources of axion dark radiation. Achieving a sufficiently strong axion flux to induce a detectable electromagnetic signal seems unlikely. Published by the American Physical Society2024 

Wave-like dark matter made of spin-1 particles (dark photons) is expected to form ground state clumps called “vector solitons”, which can have different polarizations. In this work, we consider the interaction of dark photons with photons, expressed as dimension-6 operators, and study the electromagnetic radiation that arises from an isolated vector soliton due to parametric resonant amplification of the ambient electromagnetic field. We characterize the directional dependence and polarization of the outgoing radiation, which depends on the operator as well as the polarization state of the underlying vector soliton. We discuss the implications of this radiation for the stability of solitons and as a possible channel for detecting mergers of vector solitons through astrophysical observations.