How loud are echoes from exotic compact objects?

TL;DR

This paper estimates the detectability of gravitational wave echoes from exotic compact objects with reflective surfaces, highlighting the potential for future detections with improved LIGO sensitivity.

Contribution

It provides the first estimates of the detectability of gravitational wave echoes from ECOs using a perturbative waveform model and realistic reflectivity assumptions.

Findings

Strongest echoes from comparable mass binaries

Detection of the first echo requires SNR of 20-60

Upcoming LIGO runs could observe echoes annually

Abstract

The first direct observations of gravitational waves (GWs) by the LIGO collaboration have motivated different tests of General Relativity (GR), including the search for extra pulses following the GR waveform for the coalescence of compact objects. The motivation for these searches comes from the alternative proposal that the final compact object could differ from a black hole (BH) by the lack of an event horizon and a central singularity. Such objects are expected in theories that, motivated by quantum gravity modifications, predict horizonless objects as the final stage of gravitational collapse. In such a hypothetical case, this exotic compact object (ECO) will present a (partially) reflective surface at $r_{\rm ECO}=r_{+}(1+\epsilon)$, instead of an event horizon at $r_{+}$. For this class of objects, an in-falling wave will not be completely lost and will give rise to secondary pulses, to which recent literature refers as echoes. However, the largely unknown ECO reflectivity is determinant for the amplitude of the signal, and details also depend on the initial conditions of the progenitor compact binary. Here, for the first time, we obtain estimates for the detectability of the first echo, using a perturbative description for the inspiral-merger-ringdown waveform and a physically-motivated ECO reflectivity. Binaries with comparable masses will have a stronger first echo, improving the chances of detection. For a case like GW150914, the detection of the first echo will require a minimum ringdown signal-to-noise ratio (SNR) in the range $\sim 20-60$. The most optimistic scenario for echo detection could already be probed by LIGO in the next years. With the expected improvements in sensitivity we estimate one or two events per year to have the required SNR for the first echo detection during O4.