Relativistic Poynting Jets from Accretion Disks

Abstract

A model is derived for relativistic Poynting jets from the inner region of a disk around a rotating black hole that is initially threaded by a dipole-like magnetic field. The model is derived from the special relativistic equation for a force-free electromagnetic field. The "head" of the Poynting jet is found to propagate outward with a velocity that may be relativistic. The Lorentz factor of the head is Γ = [Bo2/(8πR) 2ρextc2]1/6 if this quantity is much larger than unity. For conditions pertinent to an active galactic nuclei, Γ ≈ 8(10/R)1/3(Bc/102 G) 1/3(next/1 cm-3)-1/6, where B o is the magnetic field strength close to the black hole, ρ ext = m̄next is the mass density of the ambient medium into which the jet propagates, R = r0/rg > 1, where rg is the gravitational radius of the black hole, and r o is the radius of the O-point of the initial dipole field. This model offers an explanation for the observed Lorentz factors of ∼ 10 of parsec-scale radio jets measured with very long baseline interferometry.