Echoes of ECOs: gravitational-wave signatures of exotic compact objects and of quantum corrections at the horizon scale

TL;DR

This paper investigates gravitational-wave signatures of exotic compact objects and quantum horizon corrections, revealing universal echoes and potential observational signatures that distinguish them from black holes in binary coalescences.

Contribution

It extends the analysis of gravitational-wave echoes to a broad class of exotic objects, studies the late-time ringdown in detail, and compares boson star collisions with black-hole mergers.

Findings

Echoes are universal features in the ringdown of exotic compact objects.

Boson star collisions can mimic black-hole signals but also show distinctive signatures.

Late-time gravitational-wave signals are modulated and distorted, serving as potential observational signatures.

Abstract

Gravitational waves from binary coalescences provide one of the cleanest signatures of the nature of compact objects. It has been recently argued that the post-merger ringdown waveform of exotic ultracompact objects is initially identical to that of a black-hole, and that putative corrections at the horizon scale will appear as secondary pulses after the main burst of radiation. Here we extend this analysis in three important directions: (i) we show that this result applies to a large class of exotic compact objects with a photon sphere for generic orbits in the test-particle limit; (ii) we investigate the late-time ringdown in more detail, showing that it is universally characterized by a modulated and distorted train of "echoes" of the modes of vibration associated with the photon sphere; (iii) we study for the first time equal-mass, head-on collisions of two ultracompact boson stars and compare their gravitational-wave signal to that produced by a pair of black-holes. If the initial objects are compact enough as to mimic a binary black-hole collision up to the merger, the final object exceeds the maximum mass for boson stars and collapses to a black-hole. This suggests that - in some configurations - the coalescence of compact boson stars might be almost indistinguishable from that of black-holes. On the other hand, generic configurations display peculiar signatures that can be searched for in gravitational-wave data as smoking guns of exotic compact objects.