Kerr Black Hole in a Uniform Bertotti-Robinson Magnetic Field: An Exact Solution

A new class of exact spacetimes in Einstein's gravity, which are Kerr black holes immersed in an external uniform magnetic (or electric) field that is oriented along the rotational axis, is presented. These are axisymmetric stationary solutions to the Einstein-Maxwell equations such that (unlike in the Plebański-Demiański spacetime) the null directions of the Faraday tensor are not aligned with either of the two principal null directions of the Weyl tensor of algebraic type D (unlike the Kerr-Melvin spacetime). Three physical parameters are the black hole mass m, its rotation a, and the external field value B. For vanishing B the metric directly reduces to standard Boyer-Lindquist form of the Kerr black hole, while for zero m we recover a conformally flat Bertotti-Robinson universe with a uniform Maxwell field. For zero a, the spacetime is contained in the Van den Bergh-Carminati solutions which can be understood as the Schwarzschild black hole in a magnetic field. Our family of black holes with nonaligned Maxwell hair-that can be called the Kerr-Bertotti-Robinson black holes-may find application in various studies ranging from mathematical relativity to relativistic astrophysics.