Abstract
The most promising indirect search for the existence of axion dark matter uses radio telescopes to look for narrow spectral lines generated from the resonant conversion of axions in the magnetospheres of neutron stars. Unfortunately, a large list of theoretical uncertainties has prevented this search strategy from being fully accepted as robust. In this work we attempt to address major outstanding questions related to the role and impact of the plasma, including: $(i)$ does refraction and reflection of radio photons in the magnetosphere induce strong inhomogeneities in the flux, $(ii)$ can refraction induce premature axion-photon de-phasing, $(iii)$ to what extent do photon-plasma interactions induce a broadening of the spectral line, $(iv)$ does the flux have a strong time dependence, and $(v)$ can radio photons sourced by axions be absorbed by the plasma. We present an end-to-end analysis pipeline based on ray-tracing that exploits a state-of-the-art auto-differentiation algorithm to propagate photons from the conversion surface to asymptotically large distances. Adopting a charge symmetric Goldreich-Julian model for the magnetosphere, we show that for reasonable parameters one should expect a strong anisotropy of the signal, refraction induced axion-photon de-phasing, significant line-broadening, a variable time-dependence of the flux, and, for large enough magnetic fields, anisotropic absorption. Our simulation code is flexible enough to serve as the basis for follow-up studies with a large range of magnetosphere models.
Highlights
The QCD axion has emerged in recent years as one of the most compelling candidates to explain dark matter; this is predominantly because the axion is a fundamental ingredient in the most favored solution to the strong CP problem [1,2,3,4], which attempts to explain why CP appears to be very accurately conserved in QCD
In this work we attempt to address major outstanding questions related to the role and impact of the plasma, including: (i) does refraction and reflection of radio photons in the magnetosphere induce strong inhomogeneities in the flux, (ii) can refraction induce premature axionphoton dephasing, (iii) to what extent do photon-plasma interactions induce a broadening of the spectral line, (iv) does the flux have a strong time dependence, and (v) can radio photons sourced by axions be absorbed by the plasma
Previous attempts to compute the radio flux arising from axion-photon conversion in magnetospheres have all but ignored the complicated electrodynamics detailing how photons sourced near the neutron star surface propagate to the light cylinder3 [39,40,41,42,43]
Summary
The QCD axion has emerged in recent years as one of the most compelling candidates to explain dark matter; this is predominantly because the axion is a fundamental ingredient in the most favored solution to the strong CP problem [1,2,3,4], which attempts to explain why CP appears to be very accurately conserved in QCD. There is promise, that these uncertainties may be largely mitigated in the near future, as upcoming observations from telescopes such as SKA [51,52,53,54,55,56,57] and advances in the simulations of magnetospheres push our understanding to new limits (see, e.g., [58,59,60,61,62,63,64] for recent developments in magnetosphere modeling) The latter uncertainty, encompassing open questions such as: how axions and photons mix in an inhomogeneous three-dimensional plasma; how photons propagate through the magnetosphere; if photonplasma interactions induce a broadening of the spectral line; and if these photons can be absorbed, is more readily addressable given the current knowledge of the community. We emphasize that this work contains a complete pipeline capable of computing the expected radio signature arising from axion-photon conversion in any magnetosphere model with arbitrary axion phase space distributions, and represents a major step toward ensuring the reliability of indirect axion searches
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