On the accretion-induced QNM excitation of a Schwarzschild black hole

TL;DR

This study combines nonlinear hydrodynamics with linear perturbation equations to analyze gravitational waves emitted during matter accretion onto a Schwarzschild black hole, revealing complex interference patterns and limited QNM detectability.

Contribution

It provides a detailed analysis of gravitational wave excitation and interference effects during accretion, highlighting the limited role of QNMs in realistic scenarios.

Findings

Black hole ringdown is not a simple superposition of QNMs during accretion.

Fundamental QNM mode often dominates the gravitational-wave signal.

QNM contributions are likely too small to detect in typical astrophysical accretion events.

Abstract

By combining the numerical solution of the nonlinear hydrodynamics equations with the solution of the linear inhomogeneous Zerilli-Moncrief and Regge-Wheeler equations we investigate the properties of the gravitational radiation emitted during the axisymmetric accretion of matter onto a Schwarzschild black hole. The matter models considered include quadrupolar dust shells and thick accretion disks, permitting us to simulate situations which may be encountered at the end stages of stellar gravitational collapse or binary neutron star merger. We focus on the interference pattern appearing in the energy spectra of the emitted gravitational waves and on the amount of excitation of the quasi-normal modes of the accreting black hole. We show that, quite generically in the presence of accretion, the black hole ringdown is not a simple superposition of quasi-normal modes, although the fundamental mode is usually present and often dominates the gravitational-wave signal. We interpret this as due to backscattering of waves off the non-exponentially decaying part of the black-hole potential and to the finite spatial extension of the accreting matter. Our results suggest that the black-hole QNM contributions to the full gravitational-wave signal should be extremely small and possibly not detectable in generic astrophysical scenarios involving the accretion of extended distributions of matter.