On gravitational echoes from ultracompact exotic stars

TL;DR

This paper investigates whether ultracompact exotic stars can produce gravitational echoes, finding that physical equations of state prevent echoes but certain exotic configurations with external light rings could influence gravitational wave signals, especially when rotating.

Contribution

It demonstrates that physically supported ultracompact stars do not generate gravitational echoes, but identifies exotic configurations with external light rings that could affect gravitational wave ringdown signals, including in rotating cases.

Findings

Ultracompact stars with physical EoS do not produce gravitational echoes.

Exotic objects with external light rings can trap gravitational radiation.

Rotating exotic stars are stable against ergoregion instability.

Abstract

At the dawn of a golden age for gravitational wave astronomy, we must leave no stone unturned in our quest for new phenomena beyond our current understanding of General Relativity (GR), particle physics and nuclear physics. In this paper we discuss gravitational echoes from ultracompact stars. We restrict our analysis to exact solutions of Einstein field equations in GR that are supported by physically motivated equations of state (EoS), and in particular we impose the constraint of causality. Our main conclusion is that ultracompact objects supported by physical EoS are not able to generate gravitational echoes like those that characterize the relaxation phase of a putative black hole mimicker. Nevertheless, we identify a class of physical exotic objects that are compact enough to accommodate the presence of an external unstable light ring, thus opening the possibility of trapping gravitational radiation and affecting the ringdown phase of a merger event. Most importantly, we show that once rotation is included these stars -- contrary to what usually expected for ultracompact objects -- are not plagued by any ergoregion instability. We extend our analysis for arbitrary values of angular velocity up to the Keplerian limit, and we comment about potential signals relevant for gravitational wave interferometers.