Electromagnetic instability of compact axion stars

Abstract

If the dark matter is composed of axions, then axion stars are expected to be abundant in the Universe. We demonstrate in fully nonlinear (3+1) numerical relativity the instability of compact axion stars due to the electromagnetic Chern-Simons term. We show that above the critical coupling constant gaγcrit∝Ms-1.35, compact axion stars of mass Ms are unstable. The instability is caused by parametric resonance between the axion and the electromagnetic field. The existence of stable compact axion stars requires approximately Planck-suppressed couplings to photons. If the coupling exceeds the critical value, then all stable axion stars are necessarily noncompact. Unstable axion stars decay leaving behind a less massive, less compact, remnant. The emitted radiation peaks at a frequency ω∼1/Rs, where Rs is the axion star radius.