Multimessenger Signals from Axion Stars in Numerical Relativity

We demonstrate in fully non-linear (3+1) dimensional numerical relativity the instability of compact axion stars due to the electromagnetic Chern-Simons term. The critical coupling constant is given by gay= 12.1 in simulation units for an axion star of mass, with larger couplings being unstable. The instability shows evidence of being due to parametric resonance, in which case the critical coupling was predicted by Levkov et al to scale with axion star mass. 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 non-compact. Unstable axion stars decay leaving behind a less massive, less compact, remnant. The emitted radiation peaks at frequency 1/Rs, where Rs is the axion star radius, and shows evidence of birefringence in polarised emission.





Axion dark matter can form stable, self-gravitating, and coherent configurations known as axion stars, which are rendered unstable above a critical mass by the Chern-Simons coupling to electromagnetism. We study, using numerical relativity, the merger and subsequent decay of compact axion stars. We show that two sub-critical stars can merge, and form a more massive, excited and critical star, which survives for a finite period before rapidly decaying via electromagnetic radiation.
We find a rich multimessenger signal, composed of gravitational waves, electromagnetic radiation, and axion radiation. The gravitational wave signal is broken into two parts: a weak and broad signal from the merger, followed by a much stronger signal of almost fixed frequency from the decay. The electromagnetic radiation follows only the gravitational waves from the decay, while the axion signal is continuous throughout the process. We briefly discuss the detectability of such a signal.