We discuss axion dark matter detection via two mechanisms: spontaneous decays and resonant conversion in neutron star magnetospheres. For decays, we show that the brightness temperature signal, rather than flux, is a less ambiguous measure for selecting candidate objects. This is owing principally to the finite beam width of telescopes which prevents one from being sensitive to the total flux from the object. With this in mind, we argue that the large surface-mass-density of the galactic center or the Virgo cluster center offer the best chance of improving current constraints on the axion-photon coupling via spontaneous decays. For the neutron star case, we first carry out a detailed study of mixing in magnetized plasmas. We derive transport equations for the axion-photon system via a controlled gradient expansion, allowing us to address inhomogeneous mass-shell constraints for arbitrary momenta. We then derive a nonperturbative Landau-Zener formula for the conversion probability valid across the range of relativistic and nonrelativistic axions and show that the standard perturbative resonant conversion amplitude is a truncation of this result in the nonadiabatic limit. Our treatment reveals that infalling dark matter axions typically convert nonadiabatically in magnetospheres. We describe the limitations of one-dimensional mixing equations and explain how three-dimensional effects activate new photon polarizations, including longitudinal modes and illustrate these arguments with numerical simulations in higher dimensions. We find that the bandwidth of the radio signal from neutron stars could be dominated by Doppler broadening from the oblique rotation of the neutron star if the axion is nonrelativistic in the conversion region. Therefore, we conclude that the radio signal from the resonant decay is weaker than previously thought, which means one relies on local density peaks to probe weaker axion-photon couplings.

• Received 31 October 2019
• Accepted 11 June 2020

DOI:https://doi.org/10.1103/PhysRevD.102.023504