# Item

ITEM ACTIONSEXPORT

Released

Journal Article

#### Quasinormal modes and their overtones at the common horizon in a binary black hole merger

##### MPS-Authors

##### External Resource

No external resources are shared

##### Fulltext (restricted access)

There are currently no full texts shared for your IP range.

##### Fulltext (public)

2010.15186.pdf

(Preprint), 3MB

PhysRevD.103.044054.pdf

(Publisher version), 3MB

##### Supplementary Material (public)

There is no public supplementary material available

##### Citation

Mourier, P., Forteza, X. J., Pook-Kolb, D., Krishnan, B., & Schnetter, E. (2021).
Quasinormal modes and their overtones at the common horizon in a binary black hole merger.* Physical
Review D,* *103*(4): 044054. doi:10.1103/PhysRevD.103.044054.

Cite as: https://hdl.handle.net/21.11116/0000-0008-2C7B-E

##### Abstract

It is expected that all astrophysical black holes in equilibrium are well

described by the Kerr solution. Moreover, any black hole far away from

equilibrium, such as one initially formed in a compact binary merger or by the

collapse of a massive star, will eventually reach a final equilibrium Kerr

state. At sufficiently late times in this process of reaching equilibrium, we

expect that the black hole is modeled as a perturbation around the final state.

The emitted gravitational waves will then be damped sinusoids with frequencies

and damping times given by the quasi-normal mode spectrum of the final Kerr

black hole. An observational test of this scenario, often referred to as black

hole spectroscopy, is one of the major goals of gravitational wave astronomy.

It was recently suggested that the quasi-normal mode description including the

higher overtones might hold even right after the remnant black hole is first

formed. At these times, the black hole is expected to be highly dynamical and

non-linear effects are likely to be important. In this paper we investigate

this remarkable scenario in terms of the horizon dynamics. Working with high

accuracy simulations of a simple configuration, namely the head-on collision of

two non-spinning black holes with unequal masses, we study the dynamics of the

final common horizon in terms of its shear and its multipole moments. We show

that they are indeed well described by a superposition of ringdown modes as

long as a sufficiently large number of higher overtones are included. This

description holds even for the highly dynamical final black hole shortly after

its formation. We discuss the implications and caveats of this result for black

hole spectroscopy and for our understanding of the approach to equilibrium.

described by the Kerr solution. Moreover, any black hole far away from

equilibrium, such as one initially formed in a compact binary merger or by the

collapse of a massive star, will eventually reach a final equilibrium Kerr

state. At sufficiently late times in this process of reaching equilibrium, we

expect that the black hole is modeled as a perturbation around the final state.

The emitted gravitational waves will then be damped sinusoids with frequencies

and damping times given by the quasi-normal mode spectrum of the final Kerr

black hole. An observational test of this scenario, often referred to as black

hole spectroscopy, is one of the major goals of gravitational wave astronomy.

It was recently suggested that the quasi-normal mode description including the

higher overtones might hold even right after the remnant black hole is first

formed. At these times, the black hole is expected to be highly dynamical and

non-linear effects are likely to be important. In this paper we investigate

this remarkable scenario in terms of the horizon dynamics. Working with high

accuracy simulations of a simple configuration, namely the head-on collision of

two non-spinning black holes with unequal masses, we study the dynamics of the

final common horizon in terms of its shear and its multipole moments. We show

that they are indeed well described by a superposition of ringdown modes as

long as a sufficiently large number of higher overtones are included. This

description holds even for the highly dynamical final black hole shortly after

its formation. We discuss the implications and caveats of this result for black

hole spectroscopy and for our understanding of the approach to equilibrium.