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

TL;DR

This paper investigates how current and future radio interferometry arrays can differentiate black holes from horizonless spacetimes by analyzing indirect imprints in horizon-scale images, such as excess central brightness.

Contribution

It introduces a method to identify horizonless spacetimes through indirect image features, expanding the observational capabilities of Event Horizon Telescope arrays.

Findings

Future arrays can detect indirect imprints of horizonless spacetimes.

Second set of photon rings causes excess central brightness.

Current arrays are unable to resolve the photon rings directly.

Abstract

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 detect such indirect imprints.