The generalized second law for Kerr-Taub-NUT black holes perturbed by test field

Abstract

We evaluate the variations in the area and the entropy of Kerr-Taub-NUT black holes perturbed by bosonic and fermionic test fields. We demonstrate that the critical frequency for a test field at which the variation of the area vanishes is strictly larger than the superradiance limit for Kerr-Taub-NUT black holes. Therefore both bosonic fields in the relevant range and fermionic fields which do not exhibit superradiant scattering can induce a decrease in the area of a Kerr-Taub-NUT black hole. However, the entropy is not directly proportional to the area for Kerr-Taub-NUT black holes. Only fermionic fields with frequencies below the superradiance limit can induce an entropy decrease. The simultaneous loss of entropy by the black hole and the environment identifies a generic violation of the Generalized Second Law (GSL) for fermionic perturbations that fail to satisfy the Null Energy Condition (NEC). For bosonic perturbations that satisfy the NEC, we evaluate the limiting case, where a nearly extremal black hole is perturbed by a scalar field with frequency slightly above the superradiance limit. Our analysis reveals that the minimum increase in the black hole entropy compensates for the entropy loss of the environment, when one assigns Von Neumann entropy to the test field. The GSL remains valid for perturbations satisfying the NEC.

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