Disc-jet coupling in black hole accretion systems - I. General relativistic magnetohydrodynamical models

Abstract

General relativistic numerical simulations of magnetized accretion flows around black holes show a disordered electromagnetic structure in the disk and corona and a highly relativistic, Poynting-dominated funnel jet in the polar regions. The polar jet is nearly consistent with the stationary paraboloidal Blandford-Znajek model of an organized field threading the polar regions of a rotating black hole. How can a disordered accretion disk and corona lead to an ordered jet? We show that the polar jet is associated with a strikingly simple angular-integrated toroidal current distribution $dI_\phi/dr \propto r^{-5/4}$, where $I_\phi(r)$ is the toroidal current enclosed inside radius $r$. We demonstrate that the poloidal magnetic field in the simulated jet agrees well with the force-free field solution for a non-rotating thin disk with an $r^{-5/4}$ toroidal current, suggesting rotation leads to negligible self-collimation. We find that the polar field is confined/collimated by the corona. The electromagnetic field in the disk also scales as $r^{-5/4}$, which is consistent with some Newtonian accretion models that assume rough equipartition between magnetic and gas pressure. However, the agreement is accidental since toward the black hole the magnetic pressure increases faster than the gas pressure. This field dominance near the black hole is associated with magnetic stresses that imply a large effective viscosity parameter $\alpha\sim 1$, whereas the typically assumed value of $\alpha\sim 0.1$ holds far from the black hole.[abridged]