Low-finesse scattering and non-stationary dispersive dynamics of gravitational wave echoes

TL;DR

This paper investigates gravitational wave echoes caused by weak potential barriers outside black holes, emphasizing their non-stationary, dispersive scattering nature and providing analytical models for their transient signals.

Contribution

It introduces a novel analytical framework for modeling low-finesse gravitational wave echoes as non-stationary transient scattering phenomena, moving beyond steady state assumptions.

Findings

Echoes are governed by transient wave packets rather than steady cavity modes.

Analytical criteria for the breakdown of steady state resonance are established.

A five-parameter template accurately models early low-finesse echoes.

Abstract

We study environmental echoes induced by a weak potential barrier outside a Schwarzschild black hole. In the low-finesse limit, the time domain response is governed by a sequence of transient wave packets formed by finite round-trip scattering, rather than steady state cavity modes. We establish quantitative criteria for the breakdown of the steady state resonance picture, dictated by frequency domain spectral aliasing and time domain truncation from the black hole power law tail. Based on non-stationary dispersive dynamics, we analytically derive the arrival time gliding, central frequency drift, and dispersion driven asymmetric tails of these echoes. Accordingly, we construct a five-parameter analytical template that approaches the theoretical maximum matching degree bounded by the exact transfer function for the first echo. Our results demonstrate that early low-finesse environmental echoes must be theoretically modeled as non-stationary transient scattering signals.