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Gravitational waves from binary coalescences provide one of the cleanestsignatures of the nature of compact objects. It has been recently argued thatthe post-merger ringdown waveform of exotic ultracompact objects is initiallyidentical to that of a black-hole, and that putative corrections at the horizonscale will appear as secondary pulses after the main burst of radiation. Herewe extend this analysis in three important directions: (i) we show that thisresult applies to a large class of exotic compact objects with a photon spherefor generic orbits in the test-particle limit; (ii) we investigate thelate-time ringdown in more detail, showing that it is universally characterizedby a modulated and distorted train of "echoes" of the modes of vibrationassociated with the photon sphere; (iii) we study for the first timeequal-mass, head-on collisions of two ultracompact boson stars and comparetheir gravitational-wave signal to that produced by a pair of black-holes. Ifthe initial objects are compact enough as to mimic a binary black-holecollision up to the merger, the final object exceeds the maximum mass for bosonstars and collapses to a black-hole. This suggests that - in someconfigurations - the coalescence of compact boson stars might be almostindistinguishable from that of black-holes. On the other hand, genericconfigurations display peculiar signatures that can be searched for ingravitational-wave data as smoking guns of exotic compact objects.

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