Radio Killed the Axion Star: Constraining Axion Properties with Radio Telescopes

Abstract

Axion dark matter or any ultralight bosonic dark matter can go through Bose-Einstein condensation due to the large phase density, leading to the formation of axion stars or solitons in dark matter halo centers. The formation rate is enhanced in the presence of the substructures expected in the post-inflationary scenario for the QCD axion or axion-like particles. An axion star will continue to grow until a critical mass is reached, after which it collapses and then explodes, with the emission of relativistic axions, in a process called an ``axinovae.'' There can also be accompanying photon emission due to the stimulated decay of axions in the coherent compact axion star. In axion models with a modest enhancement ($\kappa\sim \mathcal{O}(10)$) of the axion-photon coupling $g_{a\gamma}= \kappa \alpha/(2\pi f_a)$ axinovae will contain a significant flux of radio photons. We determine the range of parameters over which axinovae can be detectable with radio transient searches.