  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.
