Shadowless rapidly rotating yet not ultraspinning Kerr-AdS$_4$ and Kerr-Newman-AdS$_4$ black holes

TL;DR

This paper investigates conditions under which Kerr-(Newman)-AdS4 black holes become shadowless due to high rotation, linking shadow properties to null hypersurface caustics and clarifying their relation to super-entropicity.

Contribution

It reveals that shadowless black holes occur beyond a critical rotation, and clarifies the independence of null hypersurface caustics from super-entropicity.

Findings

Black holes become shadowless when rotation exceeds a critical value.

Shadowless-ness is related to null hypersurface caustics inside and outside horizons.

Super-entropicity is unrelated to the presence of null hypersurface caustics.

Abstract

We find that the Kerr-(Newman)-AdS$_4$ black hole will be shadowless if its rotation parameter is larger than a critical value and the shadowless-ness may be related to the appearance of the null hypersurface caustics (NHC) both inside the Cauchy horizon and outside the event horizon for the black hole with the rotation parameter beyond the critical value. Our studies also further confirm that whether an ultraspinning black hole is super-entropic or not is unrelated to the existence of the NHC outside the event horizon.