Astrophysical haloscopes

We compute the fluxes of radio photons from conversion of axionlike particle dark matter in cosmic magnetic fields. We find that for axionlike particle masses around ${10}^{-6}\mathrm{eV}$ and effective coupling constants to photons $g_{a\gamma }\gtrsim {10}^{-13}{\mathrm{GeV}}^{-1}$ strongly magnetized nearby stellar winds may give detectable linelike 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 axionlike dark matter such observations may in turn provide information on the small scale magnetic field structure in such objects.