Impact of multiple modes on the evolution of self-interacting axion condensate around rotating black holes

TL;DR

This study investigates how multiple modes and self-interactions influence the evolution of axion condensates around rotating black holes, revealing conditions that prevent or allow explosive phenomena like bosenova.

Contribution

It provides the first detailed numerical analysis of mode interactions in axion condensates, showing their impact on superradiant instability saturation and potential for explosive events.

Findings

Mode interactions can saturate superradiant growth, preventing bosenova.

Condensates tend to settle into a two-mode quasi-stationary state.

Higher initial angular modes may lead to bosenova due to weak dissipation.

Abstract

Ultra-light particles, such as axions, form a macroscopic condensate around a highly spinning black hole by the superradiant instability. Due to its macroscopic nature, the condensate opens the possibility of detecting the axion through gravitational wave observations. However, the precise evolution of the condensate must be known for the actual detection. For future observation, we numerically study the influence of the self-interaction, especially interaction between different modes, on the evolution of the condensate in detail. First, we focus on the case when condensate starts with the smallest possible angular quantum number. For this case, we perform the non-linear calculation and show that the dissipation induced by the mode interaction is strong enough to saturate the superradiant instability, even if the secondary cloud starts with quantum fluctuations. Our result indicates that explosive phenomena such as bosenova do not occur in this case. We also show that the condensate settles to a quasi-stationary state mainly composed of two modes, one with the smallest angular quantum number for which the superradiant instability occurs and the other with the adjacent higher angular quantum number. We also study the case when the condensate starts with the dominance of the higher angular quantum number. We show that the dissipation process induced by the mode coupling does not occur for small gravitational coupling. Therefore, bosenova might occur in this case.