Radio signals from axion star-neutron star binaries

Axion stars could form binaries with neutron stars. Given the extremely strong external magnetic field exhibited by individual neutron stars, there can be a substantial conversion of axions to photons in these binaries. The photon emission is doubly modulated due to the neutron star spinning and the axion star orbiting, yielding a unique discovery signal. Similar features are also generated in binaries between a neutron star and an axion-clouded black hole. Encouragingly, such binaries are found to be within the reach of ongoing and upcoming experiments (e.g., the Five-hundred-meter Aperture Spherical Telescope and the future Square Kilometer Array) for certain parameter regions. They thus provide a promising astronomical laboratory for detecting axions and axion dark matter.