Collapse of Axion Stars

TL;DR

This paper investigates the collapse of axion stars, showing that higher-order interactions prevent black hole formation and lead to rapid axion emission during collapse.

Contribution

It demonstrates that including full potential interactions stabilizes collapsing axion stars, preventing black hole formation and revealing energy loss via relativistic axion emission.

Findings

Higher-order interactions stabilize collapsing axion stars.

Collapse results in rapid emission of relativistic axions.

Axion stars do not form black holes upon collapse.

Abstract

Axion stars, gravitationally bound states of low-energy axion particles, have a maximum mass allowed by gravitational stability. Weakly bound states obtaining this maximum mass have sufficiently large radii such that they are dilute, and as a result, they are well described by a leading-order expansion of the axion potential. Heavier states are susceptible to gravitational collapse. Inclusion of higher-order interactions, present in the full potential, can give qualitatively different results in the analysis of collapsing heavy states, as compared to the leading-order expansion. In this work, we find that collapsing axion stars are stabilized by repulsive interactions present in the full potential, providing evidence that such objects do not form black holes. In the last moments of collapse, the binding energy of the axion star grows rapidly, and we provide evidence that a large amount of its energy is lost through rapid emission of relativistic axions.