A kinetic study of black hole activation by local plasma injection into the inner magnetosphere

Abstract

ABSTRACT An issue of considerable interest in the theory of jet formation by the Blandford–Znajek mechanism, is how plasma is being supplied to the magnetosphere to maintain force-free conditions. Injection of electron–positron pairs via annihilation of MeV photons, emitted from a hot accretion flow, has been shown to be a viable possibility, but requires high enough accretion rates. At low accretion rates, and in the absence of any other form of plasma supply, the magnetosphere becomes charge-starved, forming intermittent spark gaps that can induce intense pair-cascades via interactions with disc radiation, enabling outflow formation. It is often speculated that enough plasma can penetrate the inner magnetosphere from the accretion flow through some rearrangement of magnetic field lines preventing the formation of spark-gaps. To address this question, we conducted a suite of 2D axisymmetric general-relativistic particle-in-cell simulations, in which plasma is injected into specified regions at a predescribed rate. We find that when pair-production is switched off, nearly complete screening is achieved when plasma is injected at the entire region inside the outer light cylinder at a high enough rate. Injection outside this region results in either, the formation of large vacuum-gaps, or coherent, large-amplitude oscillations of the magnetosphere, depending on the injection rate. Within our allowed dynamical range, we see no evidence for the system to reach a steady-state at high injection rates. Switching on pair-production results in nearly complete screening of the entire magnetosphere in all cases, with a small fraction of the Blandford–Znajek power dissipated as TeV gamma-rays.