A note on the observational evidence for the existence of event horizons in astrophysical black hole candidates

TL;DR

This paper reviews observational evidence for event horizons in black hole candidates by comparing constraints from electromagnetic non-detection and ergoregion instability, highlighting assumptions and limitations.

Contribution

It provides a comparative analysis of different observational constraints on the existence of event horizons in astrophysical black hole candidates.

Findings

Non-observation of electromagnetic radiation constrains surface properties.

Ergoregion instability bounds depend on specific assumptions.

Both methods support the presence of event horizons.

Abstract

Black holes have the peculiar and intriguing property of having an event horizon, a one-way membrane causally separating their internal region from the rest of the Universe. Today astrophysical observations provide some evidence for the existence of event horizons in astrophysical black hole candidates. In this short paper, I compare the constraint we can infer from the non-observation of electromagnetic radiation from the putative surface of these objects with the bound coming from the ergoregion instability, pointing out the respective assumptions and limitations.