Nonlinear treatment of a black hole mimicker ringdown

TL;DR

This paper presents the first nonlinear, self-consistent simulation of the merger and ringdown of a black hole mimicker with stable light rings, revealing gravitational wave features similar to black holes and novel echo-like signals.

Contribution

It introduces a fully nonlinear numerical study of binary boson star mergers resulting in horizonless remnants with stable light rings, advancing understanding of gravitational wave signatures.

Findings

Remnant waveforms approach Kerr black hole signals at high compactness

Detected high-frequency gravitational wave echoes from remnant perturbations

Quantified energy emitted and timescales of gravitational wave signals

Abstract

We perform the first nonlinear and self-consistent study of the merger and ringdown of a black hole mimicking object with stable light rings. To that end, we numerically solve the full Einstein-Klein-Gordon equations governing the head-on collisions of a series of binary boson stars in the large-mass-ratio regime resulting in spinning horizonless remnants with stable light rings. We broadly confirm the appearance of features in the extracted gravitational waveforms expected based on perturbative methods: the signal from the prompt response of the remnants approaches that of a Kerr black hole in the large-compactness limit, and the subsequent emissions contain periodically appearing bursts akin to so-called gravitational wave echoes. However, these bursts occur at high frequencies and are sourced by perturbations of the remnant's internal degrees of freedom. Furthermore, the emitted waveforms also contain a large-amplitude and long-lived component comparable in frequency to black hole quasi-normal modes. We further characterize the emissions, obtain basic scaling relations of relevant timescales, and compute the energy emitted in gravitational waves.