Horizonless Spacetimes As Seen by Present and Next-generation Event Horizon Telescope Arrays

We study the capabilities of present and future radio very long baseline interferometry arrays to distinguish black holes from horizonless spacetimes. We consider an example of a horizonless spacetime, obtained by overspinning a regular black hole. Its image is distinct from the image of a Kerr spacetime due to a second set of photon rings interior to the shadow. These photon rings cannot be directly resolved by present and even next-generation Event Horizon Telescope arrays, but instead imprint themselves in horizon-scale images as excess central brightness relative to that of a black hole. We demonstrate that future arrays can decrease the upper bound on the central brightness depression by a factor of 10 and thereby detect such indirect imprints from non-Kerr spacetimes in reconstructed images.