Plasma Cascade in Kerr Black Hole Magnetospheres

Abstract

Electromagnetic, radiative, and plasma processes around black holes in active galaxies determine how relativistic jets are launched and the efficiency at which the black hole energy is extracted via the Blandford-Znajek mechanism, which converts the black hole rotational energy into Poynting flux. The crucial assumption is the force-free condition, which is the presence of plasma with a density at or above the Goldreich-Julian density. Unlike neutron stars, which in principle can supply electrons from their surface, black holes cannot supply plasma at all, they are only a sink. Therefore, the plasma needed must be generated in situ. The essential process is the plasma production via an electron-position cascade in the so-called “gap” region in the force-free magnetosphere around the black hole. This multi-stage process, involving particle acceleration, photon Compton up-scattering, and production of electron-positron secondaries, is explored numerically by computing the radial development of the entire cascade. It is shown how the electron-positron plasma production depends on the black hole mass and spin, the energy density of the ambient photons, and seed magnetic field strength. Presented is the full, two-dimensional structure of the gap, along with empirical scaling relations for the two-dimensional gap structure. Observational predictions for X-ray and γ-ray fluxes and spectra, which can be compared with observations of the inner regions near jets and estimations of the structure of the gaps in several galaxies, e.g., Messier 87, using the empirical scaling relations are discussed.