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 Gamma = [B-0(2)/(8piR)(2) rho(ext)C(2)](1/6) if this quantity is much larger than unity. For conditions pertinent to an active galactic nuclei, Gamma approximate to 8(10/R)(1/3) (B-0/10(3) G)(1/3) (eta(ext)/1 cm(-3))(-1/6), where B-0 is the magnetic field strength close to the black hole, rho(ext) = (m) over barn(ext) is the mass density of the ambient medium into which the jet propagates, R = r(o)/r(g) > 1, where r(g) 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 similar to 10 of parsec-scale radio jets measured with very long baseline interferometry.