Unruh-DeWitt detector differentiation of black holes and exotic compact objects

We study the response of a static Unruh-DeWitt detector outside an exotic compact object (ECO) in 3+1 dimensions. The horizonless ECO, whose surface is extremely close to the would-be event horizon, acts as a black hole mimicker. ECOs that have an origin provide a physical means by which all waves, including gravitational waves, are reflected (off the origin). They also provide a source of damping of a wave as it traverses the material in the ECO. We study here the case of a probe scalar field for simplicity. We find that the response rate is notably distinct from the black hole case, even when the wave is lost completely to damping. For partial or no damping we find resonance structures in the response rate that depend on the compactness of the ECO and the detector location. We provide a detailed analysis in connection with the ECO's vacuum mode structure and the transfer function.