Relativistic Poynting Jets from Accretion Disks

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 = [B/(8π)²ρ_extc²]^1/6 if this quantity is much larger than unity. For conditions pertinent to an active galactic nuclei, Γ ≈ 8(10/)^1/3(B₀/10³ G)^1/3(n_ext/1 cm^-3)^-1/6, where B₀ is the magnetic field strength close to the black hole, ρ_ext = n_ext is the mass density of the ambient medium into which the jet propagates, = r₀/r_g > 1, where r_g is the gravitational radius of the black hole, and r₀ 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.