Contours of constant pitch angle (χ) of electrons moving in the electromagnetic field of the Papapetrou-Wald solution shown in Fig. 1. For the present magnetic dominated case (|E|/|B|< 1), charged particles follow the magnetic field lines; therefore, (see, e.g., Kelner et al. 2015), where E_|| is the electric field component parallel to the magnetic field, and Ψ is the angle between B and −E (the minus sign is used because we are interested in the pitch angle of electrons). In the slow-rotation regime (see Eqs. (6a) and (6b)), sin Ψ ≈ sin θ/(1−2Msin²θ/r), so sin χ ≈ |E|/|B|sin θ/(1−2Msin²θ/r). The BH is indicated by the black disk. The background color map indicates the electric field energy density (lighter colors means more intense).