Resonant conversion of axion dark radiation into terahertz electromagnetic radiation in a neutron star magnetosphere

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2024
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American Physical Society
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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 𝜔pl ≈𝑚𝑎. The Euler-Heisenberg assisted resonance occurs where the axion energy satisfies 𝜔 ≈(2⁢𝜔2pl/7⁢𝑔𝛾⁢𝛾⁢𝛾⁢𝛾⁡¯𝐵2)1/2. This second resonance is made possible though the strong background magnetic field ¯𝐵, as well as the nonzero Euler-Heisenberg four-photon self-interaction, which has the coupling 𝑔𝛾⁢𝛾⁢𝛾⁢𝛾 =8⁢𝛼2/45⁢𝑚4𝑒. 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 𝑔𝑎⁢𝛾⁢𝛾 and different amplitudes for the axion flux onto the neutron star Φ𝑎. We briefly discuss several possible sources of axion dark radiation. Achieving a sufficiently strong axion flux to induce a detectable electromagnetic signal seems unlikely.

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Long, A. J., & Schiappacasse, E. D. (2024). Resonant conversion of axion dark radiation into terahertz electromagnetic radiation in a neutron star magnetosphere. Physical Review D, 110(10), 103020. https://doi.org/10.1103/PhysRevD.110.103020

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