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A model is constructed to predict the emission originating from axion-to-photon conversion in the strongly magnetized ultrarelativistic plasma of neutron stars. The acceleration and multiplicity of the charges are observed to shift the axion-induced spectral feature with respect to previous expectations. The frequency range of interest widens accordingly, and heavier dark matter axions may resonate in magnetospheric splits giving rise to detectable radio signals that could extend into the millimeter band. Ultimately, this work follows an affirmative answer to the question of whether neutron stars can give rise to any detectable high-frequency spectral feature that would allow us to probe axion dark matter of masses up to about a millielectronvolt.

We treat prospects for multimessenger astronomy with giant flares (GFs), a rare transient event featured by magnetars that can be as luminous as a hundred of the brightest supernovae ever observed. The beamed photons could correlate with an axion counterpart via resonant conversion in the magnetosphere. In a realistic parameter space, we find that the sensitivity limit to galactic GFs for currently viable experiments is $\mathrm{g}_{\phi \gamma}\!\gtrsim\!\mathrm{several}\!\times\!10^{-13}$ GeV$^{-1}$ \& $\mathrm{g}_{\phi e}\!\gtrsim\!\mathrm{few}\!\times\!10^{-12}$. We rule out the compatibility of axion flares with the recent XENON1T excess only due to the time persistence of the signal., Comment: 7 pages, 3 figures

In this article we treat the non-adiabatic photon-to-axion resonant conversion of curvature radiation, synchrotron emission and inverse Compton scattering dominating the spectral density function of pulsars. First, we introduce emission models and benchmark observational data.vWe adopt a state-of-the-art density profile that relieves tension with the quantum electrodynamics vacuum polarization effect in highly magnetic stars, leading to efficient mixing. Then, we estimate the dark matter flux induced by photon-axion oscillation across the light cylinder of the neutron star. We find that pulsars might produce axion overdensities many orders of magnitude over the occupation number of dark matter in the Galactic halo within a broad parameter space. We point out possible new methods for axion detection derived from these results and other future lines of work., Comment: 15pp., 4 figs; as accepted for publication in JCAP