Stationary electro-vacuum fields around black holes

TL;DR

This paper reviews relativistic electrodynamics near black holes, focusing on exact solutions, electromagnetic field structures, and phenomena like magnetic reconnection and flux expulsion, highlighting new effects in Einstein-Maxwell solutions.

Contribution

It presents a pedagogical overview of exact electro-vacuum solutions around black holes, including new insights into magnetic flux expulsion and field structures near horizons.

Findings

Magnetic and electric field lines near black holes can develop null points and current sheets.

Magnetic flux expulsion depends on the magnetic field strength in exact solutions.

Electromagnetic fields near rotating black holes exhibit frame-dragging effects.

Abstract

This is the second lecture of `RAGtime' series on electrodynamical effects near black holes. We will summarize the basic equations of relativistic electrodynamics in terms of spin-coefficient (Newman-Penrose) formalism. The aim of the lecture is to present important relations that hold for exact electro-vacuum solutions and to exhibit, in a pedagogical manner, some illustrative solutions and useful approximation approaches. First, we concentrate on weak electromagnetic fields and we illustrate their structure by constructing the magnetic and electric lines of force. Gravitational field of the black hole assumes axial symmetry, whereas the electromagnetic field may or may not share the same symmetry. With these solutions we can investigate the frame-dragging effects acting on electromagnetic fields near a rotating black hole. These fields develop magnetic null points and current sheets. Their structure suggests that magnetic reconnection takes place near the rotating black hole horizon. Finally, the last section will be devoted to the transition from test-field solution to exact solutions of coupled Einstein-Maxwell equations. New effects emerge within the framework of exact solutions: the expulsion of the magnetic flux out of the black-hole horizon depends on the intensity of the imposed magnetic field.