We show that axion dark matter (DM) may be detectable through narrow radio lines emitted from neutron stars. The neutron star magnetosphere hosts a strong magnetic field and a plasma frequency that increases towards the neutron star surface. As the axions pass through the magnetosphere, they can resonantly convert into radio photons in a narrow region around the radius at which the plasma frequency equals the axion mass. The bandwidth of the signal is set by the small DM velocity dispersion far away from the neutron star. We solve the axion-photon mixing equations, including a full treatment of the magnetized plasma and associated anisotropic dielectric tensor, to obtain the conversion probability. We discuss possible neutron-star targets and how they may probe the QCD axion parameter space in the mass range of ∼0.2-40 µeV.arXiv:1804.03145v1 [hep-ph] 9 Apr 2018 for collaboration in the early stages of this project. We thank Anatoly Spitkovsky for detailed discussions regarding NS magnetospheres, and

It has been suggested that radio telescopes may be sensitive to axion dark matter that resonantly converts to radio photons in the magnetospheres surrounding neutron stars (NSs). In this work, we closely examine this possibility by calculating the radiated power from and projected sensitivity to axion dark matter conversion in ensembles of NSs within astrophysical systems like galaxies and globular clusters. We use population synthesis and evolution models to describe the spatial distributions of NSs within these systems and the distributions of NS properties. Focusing on three specific targets for illustration, the Galactic Center of the Milky Way, the globular cluster M54 in the Sagittarius dwarf galaxy, and the Andromeda galaxy, we show that narrow-band radio observations with telescopes such as the Green Bank Telescope and the future Square Kilometer Array may be able to probe the quantum chromodynamics axion over roughly two orders of magnitude in mass, starting at a fraction of a µeV.

Axion dark matter (DM) may convert to radio-frequency electromagnetic radiation in the strong magnetic fields around neutron stars. The radio signature of such a process would be an ultranarrow spectral peak at a frequency determined by the mass of the axion particle. We analyze data we collected from the Robert C. Byrd Green Bank Telescope in the L band and the Effelsberg 100-m Telescope in the L band and S band from a number of sources expected to produce bright signals of axion-photon conversion, including the Galactic center of the Milky Way and the nearby isolated neutron stars RX J0720.4-3125 and RX J0806.4-4123. We find no evidence for axion DM and are able to set constraints on the existence of axion DM in the highly motivated mass range between ∼5 and 11 μeV with the strongest constraints to date on axions in the ∼10-11 μeV range.

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