Self-interacting axion clouds around rotating black holes in binary systems

TL;DR

This paper investigates how axion clouds around rotating black holes in binary systems evolve during inspiral, highlighting the effects of self-interaction and tidal forces on gravitational wave signals and potential instabilities.

Contribution

It is the first study to analyze the evolution of axion clouds in binary systems including self-interaction effects and their impact on gravitational wave signatures.

Findings

Self-interaction modifies GW phase signatures.

Coexistence of two cloud types through mode coupling.

Potential for bosenova instability during inspiral.

Abstract

Superradiant instability can form clouds around rotating black holes (BHs) composed of ultralight bosonic fields, such as axions. A BH with such a cloud in a binary system exhibits rich phenomena, and gravitational waves (GWs) from the BH merger provide a means to probe axions. For the first time, we study the evolution of axion clouds in a binary system during the inspiral phase, including axion self-interaction effects. When the self-interaction is significant, unlike in the negligible case, two types of clouds coexist through mode coupling. We examine the evolution of the system considering the effects of dissipation caused by both self-interaction and tidal interaction. For tidal interaction, in addition to the processes of emission to infinity and absorption by the BH, indirect emission via transitions (both resonant and off-resonant) is also considered as a second-order perturbation. Our results demonstrate that the signatures of axion self-interaction are imprinted in the modification of the GW phase. Furthermore, we find the possibility of a dynamical instability called bosenova during the binary inspiral phase.