The inner Cauchy horizon of axisymmetric and stationary black holes with surrounding matter**This paper is dedicated to Reinhard Meinel on the occasion of his 50th birthday.

Abstract

We investigate the interior of regular axisymmetric and stationary black holes surrounded by matter and find that for non-vanishing angular momentum of the black hole the spacetime can always be extended regularly up to and including an inner Cauchy horizon. We provide an explicit relation for the regular metric at the inner Cauchy horizon in terms of that at the event horizon. As a consequence, we obtain the universal equality (8πJ)2 = A+A− where J is the black hole's angular momentum and A− and A+ denote the horizon areas of inner Cauchy and event horizons, respectively. We also find that in the limit J → 0 the inner Cauchy horizon becomes singular.