Electrically-charged black holes and the Blandford-Znajek mechanism

TL;DR

This paper revisits the properties of electrically charged black holes and the Blandford-Znajek mechanism, showing that previous claims of a 'dead' magnetosphere are incorrect and that black hole charge influences magnetic field behavior.

Contribution

The study clarifies the properties of Wald's charged black hole solutions and demonstrates that electric charge can affect magnetic field lines, challenging prior assumptions about black hole magnetospheres.

Findings

Electrostatic potential drops along magnetic field lines except on the symmetry axis.

Charged black holes attract magnetic field lines back onto the horizon.

The Wald charge $q_0$ is an upper limit, not a fixed value.

Abstract

Recently, it was claimed that accretion of electric charge by a black hole rotating in an aligned external magnetic field results in a "dead" vacuum magnetosphere, where the electric field is totally screened, no vacuum breakdown is possible, and the Blandford-Znajek mechanism cannot operate King & Pringle (2021). Here we study in details the properties of the Wald(1974) solution for electrically charged black holes discussed in that paper. Our results show that the claim is erroneous as in the solution with the critical charge $q_0=2aB_0$ there exists a drop of electrostatic potential along all magnetic field lines except the one coinciding with the symmetry axis. It is also found that while uncharged rotating black holes expel external vacuum magnetic field from their event horizon (the Meissner effect), electric charging of black holes pulls the magnetic field lines back on it, resembling what has been observed in some previous force-free, RMHD and PIC simulations of black hole magnetospheres. This suggests that accretion of electric charge may indeed be a feature of the black hole electrodynamics. However, our analysis shows that the value $q_0$ of the BH charge given by Wald is likely to be only an upper limit, and that the actual value depends of the details of the magnetospheric physics.