Gravitational-wave echoes from the coalescence of black hole binaries are frequently viewed as signals beyond general relativity or standard model. In this work, we show that these echoes are possible even in the coalescence of standard general relativity. This is because the physical black holes formed through gravitational collapse serve as a faithful description of astro black holes. To all outside probes, only their asymptotic structure before the horizon formation is accessible. Here, we investigate the scattering of a gravitational-wave bursts on such physical black holes and pay special attention to the echoes. Our results uncover distinct features of this echo both in time and frequency domains.

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• Classical black holes
• Gravitational waves