Subdominant Modes of Scalar Superradiant Instability and Gravitational Wave Beats

TL;DR

This paper investigates the role of subdominant scalar modes in black hole superradiance, revealing their impact on gravitational wave signals and potential for distinguishing black hole systems from other sources.

Contribution

It provides a detailed analysis of subdominant modes' effects on black hole evolution and gravitational wave signatures, highlighting the interference patterns that produce detectable beats.

Findings

Subdominant modes do not alter the main black hole evolution along the Regge trajectory.

Interference between modes creates gravitational wave beats.

Beats can help distinguish black hole systems from neutron stars.

Abstract

Ultralight scalars could extract energy and angular momentum from a Kerr black hole (BH) because of superradiant instability. Multiple modes labelled with $nlm$ grow while rotating around the BH, emitting continuous gravitational wave (GW). In this work, we carefully study the contribution of the subdominant modes with $n\geq 1$ in the evolution of the BH-condensate system. We find the BH still evolves along the Regge trajectory of the $n=0$ modes even with the presence of the subdominant modes. The interference of the dominant and the subdominant modes produces beats in the emitted GW, which could be used to distinguish the BH-condensate systems from other monochromatic GW sources, such as neutron stars.