Apparent shape of super-spinning black holes

TL;DR

This paper explores how super-spinning black holes, which violate the Kerr bound, could have significantly smaller and distinct shadows, providing potential observational signatures to identify such objects.

Contribution

It introduces the concept that super-spinning black holes can alter the apparent shape of their shadows, offering a new way to detect violations of the Kerr bound.

Findings

Super-spinning black holes have much smaller shadows than Kerr black holes.

Violations of the Kerr bound can produce observable differences in black hole images.

Potential observational signatures could be detected in mm-range observations of galactic centers.

Abstract

We consider the possibility that astrophysical Black Holes (BHs) can violate the Kerr bound; i.e., they can have angular momentum greater than BH mass, $J > M$. We discuss implications on the BH apparent shape. Even if the bound is violated by a small amount, the shadow cast by the BH changes significantly (it is $\sim$ an order of magnitude smaller) from the case with $J \le M$ and can be used as a clear observational signature in the search for super--spinning BHs. We discuss briefly recent observations in the mm range of the super--massive BH at the Center of the Galaxy, speculating on the possibility that it might violate the Kerr bound.