How well can ultracompact bodies imitate black hole ringdowns?

TL;DR

This paper investigates how well ultracompact horizonless objects, like gravastars, can mimic black hole ringdowns in gravitational wave signals, providing a way to distinguish them from true black holes with current detectors.

Contribution

It introduces a novel calculation of early ringdown frequencies and damping times for rotating ultracompact objects using a third-order Hartle-Thorne approximation.

Findings

Gravastar ringdown signals can differ by a few percent from Kerr black holes.

Differences in ringdown signals are potentially detectable by near-future gravitational wave observatories.

The study provides a framework for testing the nature of compact objects through gravitational wave observations.

Abstract

The ongoing observations of merging black holes by the instruments of the fledging gravitational wave astronomy has opened the way for testing the general relativistic Kerr black hole metric and, at the same time, for probing the existence of more speculative horizonless ultracompact objects. In this paper we quantify the difference that these two classes of objects may exhibit in the post-merger ringdown signal. By considering rotating systems in general relativity and assuming an eikonal limit and a third-order Hartle-Thorne slow rotation approximation, we provide the first calculation of the early ringdown frequency and damping time as a function of the body's multipolar structure. Using the example of a gravastar, we show that the main ringdown signal may differ as much as a few percent with respect to that of a Kerr black hole, a deviation that could be probed by near future Advanced LIGO/Virgo searches.