Apparent shape of super-spinning black holes

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.