Electromagnetic instability of compact axion stars
Liina Chung-Jukko, Eugene A. Lim, David J. E. Marsh, Josu C. Aurrekoetxea, Eloy de Jong, Bo-Xuan Ge
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 ${g}_{a\ensuremath{\gamma}}^{\mathrm{crit}}\ensuremath{\propto}{M}_{s}^{\ensuremath{-}1.35}$, compact axion stars of mass ${M}_{s}$ 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 $\ensuremath{\omega}\ensuremath{\sim}1/{R}_{s}$, where ${R}_{s}$ is the axion star radius.