Greybody factors as robust gravitational observables: insights into post-merger signals and echoes from ultracompact object

Abstract: The quasinormal mode spectrum plays a central role in modeling the post-merger ringdown phase of binary coalescences of compact objects. However, its interpretation is subject to certain ambiguities. Motivated by a recently discovered connection between greybody factors and post-merger black hole signals, we investigate the robustness of greybody factors as gravitational observables, offering a complementary perspective to quasinormal mode analysis. We show that greybody factors are stable under small perturbations and are not affected by the specific ambiguities that limit the reliability of quasinormal modes. Furthermore, we demonstrate that greybody factors are equally relevant for characterizing the signals emitted by wormholes and other horizonless ultracompact objects, providing a natural explanation for the echoes observed in the time-domain response of such object.