Astrophysical Haloscopes

We compute the fluxes of radio photons from conversion of axion-like particle dark matter in cosmic magnetic fields. We find that for axion-like particle masses around ${10}^{-6}$eV and effective coupling constants to photons $g_{a\gamma }\gtrsim {10}^{-13}{\mathrm{G}\mathrm{e}\mathrm{V}}^{-1}$ strongly magnetized nearby stellar winds may give detectable line-like radio photon signals, although predicted fluxes are highly uncertain due to the poorly known structure of the magnetic fields. Nevertheless, it may be worth while to conduct a dedicated search in the direction of such sources. When combined with a possible future laboratory detection of axion-like dark matter such observations may in turn provide information on the small scale magnetic field structure in such objects.