Horizonless spacetimes as seen by present and next-generation Event Horizon Telescope arrays

Understand

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.