Thermodynamics of Kerr-Bertotti-Robinson black hole

TL;DR

This paper studies the thermodynamic properties of the Kerr-Bertotti-Robinson black hole, focusing on defining its mass and thermodynamic potentials in the presence of an external electromagnetic field.

Contribution

It introduces a novel method to define the conserved mass of the black hole using the Christodoulou-Ruffini relation, overcoming previous integrability issues.

Findings

Thermodynamic quantities satisfy the first law and Smarr formula.

A consistent definition of mass is achieved despite external electromagnetic fields.

The approach clarifies the thermodynamics of rotating black holes in external fields.

Abstract

We investigate the thermodynamic properties of the Kerr-Bertotti-Robinson black hole, an exact Petrov type D solution of Einstein-Maxwell theory describing a rotating black hole immersed in an external electromagnetic field. While the conserved angular momentum and electric charge can be computed straightforwardly, the conserved mass cannot be obtained through standard integrability methods due to the nontrivial asymptotically uniform external electromagnetic field. To overcome this difficulty, we adopt the Christodoulou-Ruffini mass relation as a thermodynamic definition of the conserved mass, and identify the associated generator, thereby fixing the ambiguity in defining this conserved mass and constructing the thermodynamic potentials. These thermodynamic quantities naturally satisfy the first law of black-hole thermodynamics as well as the Smarr formula.