Nonlinear effect of absorption on the ringdown of a spinning black hole

TL;DR

This paper investigates how nonlinear absorption effects influence the black hole ringdown signal, revealing mode excitations and early-time components that are crucial for accurate gravitational wave analysis.

Contribution

It systematically studies the nonlinear effect of absorption on ringdown signals by evolving perturbations on a dynamic black hole background, highlighting new mode excitations and early-time features.

Findings

Excitation of higher polar angular modes

Generation of retrograde and overtone modes

Presence of early-time non-quasinormal mode components

Abstract

The ringdown gravitational wave signal arising e.g., in the final stage of a black hole binary merger, contains important information about the properties of the remnant, and can potentially be used to perform clean tests of general relativity. However, interpreting the ringdown signal, in particular when it is the loudest, requires understanding the role of nonlinearities and their potential impact on modelling this phase using quasinormal modes. Here, we focus on a particular nonlinear effect arising from the change in the black hole's mass and spin due to the partial absorption of a quasinormal perturbation. We isolate and systematically study this third-order, secular effect by evolving the equations governing linear metric perturbations on the background of a spinning black hole, but allowing the properties of the background to evolve in a prescribed way. We find that this leads to the excitation of quasinormal modes with higher polar angular number, retrograde modes (counter-rotating with respect to the black hole), and overtones, as well as giving rise to a component of the signal at early times that cannot be fully described using quasinormal modes. Quantifying these effects, we find that they may be relevant in analyzing the ringdown in black hole mergers.