Ultrarelativistic black hole in an external electromagnetic field and gravitational waves in the Melvin universe

TL;DR

This paper studies the ultrarelativistic limit of a Schwarzschild black hole in a Melvin electromagnetic universe, resulting in impulsive gravitational waves and analyzing their geometric and algebraic properties.

Contribution

It derives an exact impulsive gravitational wave solution in the Melvin universe via boosting, extending previous models and classifying the wave's algebraic type and geometry.

Findings

The impulsive wave reduces to Aichelburg-Sexl pp-wave when electromagnetic field vanishes.

The wave front has non-constant Gauss curvature.

The solutions are of Petrov type II and belong to the Kundt class.

Abstract

We investigate the ultrarelativistic boost of a Schwarzschild black hole immersed in an external electromagnetic field, described by an exact solution of the Einstein-Maxwell equations found by Ernst (the ``Schwarzschild-Melvin'' metric). Following the classical method of Aichelburg and Sexl, the gravitational field generated by a black hole moving ``with the speed of light'' and the transformed electromagnetic field are determined. The corresponding exact solution describes an impulsive gravitational wave propagating in the static, cylindrically symmetric, electrovac universe of Melvin, and for a vanishing electromagnetic field it reduces to the well known Aichelburg-Sexl pp-wave. In the boosting process, the original Petrov type I of the Schwarzschild-Melvin solution simplifies to the type II on the impulse, and to the type D elsewhere. The geometry of the wave front is studied, in particular its non-constant Gauss curvature. In addition, a more general class of impulsive waves in the Melvin universe is constructed by means of a six-dimensional embedding formalism adapted to the background. A coordinate system is also presented in which all the impulsive metrics take a continuous form. Finally, it is shown that these solutions are a limiting case of a family of exact gravitational waves with an arbitrary profile. This family is identified with a solution previously found by Garfinkle and Melvin. We thus complement their analysis, in particular demonstrating that such spacetimes are of type II and belong to the Kundt class.