Adiabatic evolution of the self-interacting axion field around rotating black holes

TL;DR

This paper introduces a new adiabatic method to analyze the evolution of self-interacting axion clouds around rotating black holes, revealing how different coupling strengths influence cloud stability and the potential for explosive bosenova events.

Contribution

It presents a novel, non-perturbative adiabatic approach to track self-interacting axion cloud evolution, improving understanding of stability and explosive phenomena around black holes.

Findings

Strong coupling leads to stable, quasi-stationary clouds.

Weak coupling can cause cloud instability and bosenova.

Method does not rely on non-relativistic or perturbative approximations.

Abstract

Ultra light axion fields, motivated by the string theory, form a large condensate (axion cloud) around rotating black holes through superradiant instability. Several effects due to the axion cloud, such as the spin-down of black holes and the emission of monochromatic gravitational waves, open a new window to search for axions by astrophysical observations. When the axion self-interaction is considered, the evolution of cloud is altered significantly, and an explosive phenomenon called bosenova can happen. Thus, it is necessary to understand the precise evolution of self-interacting clouds for the detection of axions by astrophysical observations. In this paper, we propose a new method to track the whole process of the growth of self-interacting axion clouds employing the adiabatic approximation. We emphasize that our method relies neither on the non-relativistic approximation nor on perturbative treatment of the self-interaction, which is often used in literature. Our main finding is that the evolution of cloud in the strongly self-interacting regime depends on the strength of the gravitational coupling between the axion and the black hole. For a large coupling, the cloud evolves into a quasi-stationary state where the superradiant energy gain is balanced with the energy dissipation to infinity by the self-interaction. On the other hand, when one decreases the size of coupling, clouds become unstable at some energy, which would be interpreted as the onset of bosenova.