Constraints on the amplitude of gravitational wave echoes from black hole ringdown using minimal assumptions

Gravitational wave echoes may appear following a compact binary coalescence if the remnant is an ``exotic compact object.'' Exotic compact objects are proposed alternatives to the black holes of Einstein's general relativity theory and are predicted to possess reflective boundaries. This work reports a search for gravitational wave transients (GWTs) of generic morphology occurring shortly after ($\ensuremath{\lesssim}1\text{ }\text{ }\mathrm{s}$) binary black hole (BBH) mergers, therefore targeting all gravitational wave echo models. We investigated the times after the ringdown for the higher signal-to-noise ratio BBHs within the public catalog GWTC-3 by the LIGO-Virgo-KAGRA collaboration. Our search is based on the coherent WaveBurst pipeline, widely used in generic searches for GWTs by the LIGO-Virgo-KAGRA collaboration, and deploys new methods to enhance its detection performances at low signal-to-noise ratios. We employ Monte Carlo simulations for estimating the detection efficiency of the search and determining the statistical significance of candidates. We find no evidence of previously undetected GWTs and our loudest candidates are morphologically consistent with known instrumental noise disturbances. Finally, we set upper limits on the amplitude of GW echoes for single BBH mergers.