Black Hole Echology: The Observer's Manual

While recent detections of gravitational waves from the mergers of binary black holes match well with the predictions of General Relativity (GR), they cannot directly confirm the existence of event horizons. Exotic compact objects (ECOs) are motivated by quantum models of black holes, and can have exotic structure (or a "wall") just outside the (would-be) horizon. ECOs produce similar ringdown waveforms to the GR black holes, but they are followed by delayed "echoes". By solving linearized Einstein equations we can model these echoes and provide analytic templates that can be used to compare to observations. For concreteness, we consider GW150914 event, detected by the LIGO/Virgo collaboration, and study the model dependence of its echo properties. We find that echoes are reasonably approximated by complex gaussians, with amplitudes that decay as a power law in time, while their width in time (frequency) grows (shrinks) over subsequent echoes. We also show that trapped modes between a perfectly reflecting wall and angular momentum barrier in Kerr metric can exhibit superradiant instability over long times, as expected.