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

Abstract

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≈(2ωpl2/7gγγγγB¯2)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 gγγγγ=8α2/45me4. 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 gaγγ 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.