# Exotic compact object behavior in black hole analogues

## TL;DR

This paper investigates the stability and superradiant scattering of acoustic perturbations in a black hole analogue model with a reflective surface near the horizon, revealing conditions for stability and enhanced wave amplification.

## Contribution

It introduces a model of an exotic compact object with a reflective surface near the horizon and analyzes its stability and superradiant scattering properties.

## Key findings

- Stability is achievable when reflectivity is less than approximately 70%.
- Superradiant scattering is more effective near quasi-normal mode frequencies.
- High reflectivity can lead to instabilities in the system.

## Abstract

Classical phenomenological aspects of acoustic perturbations on a draining bathtub geometry where a surface with reflectivity $\mathcal{R}$ is set at a small distance from the would-be acoustic horizon, which is excised, are addressed. Like most exotic compact objects featuring an ergoregion but not a horizon, this model is prone to instabilities when $|\mathcal{R}|^2\approx 1$. However, stability can be attained for sufficiently slow drains when $|\mathcal{R}|^2\lesssim70\%$. It is shown that the superradiant scattering of acoustic waves is more effective when their frequency approaches one of the system's quasi-normal mode frequencies.