Electromagnetic instability of compact axion stars

TL;DR

This paper demonstrates that compact axion stars become electromagnetically unstable above a critical coupling, leading to decay and emission of radiation, with implications for dark matter models involving axions.

Contribution

The study provides the first non-linear numerical relativity analysis of electromagnetic instability in compact axion stars, identifying critical coupling thresholds and decay outcomes.

Findings

Unstable axion stars decay when coupling exceeds critical value.

Decay produces radiation peaking at frequency related to star radius.

Stable compact axion stars require Planck-suppressed photon couplings.

Abstract

If the dark matter is composed of axions, then axion stars are expected to be abundant in the Universe. We demonstrate in fully non-linear (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 $g_{a\gamma}^\mathrm{crit} \propto M_s^{-1.35}$, 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 non-compact. Unstable axion stars decay leaving behind a less massive, less compact, remnant. The emitted radiation peaks at frequency $\omega \sim 1/R_s$, where $R_s$ is the axion star radius.