Relativistic Outflows in Active Galactic Nuclei

There are observational indications that relativistic outflows in AGNs are accelerated over distances that far exceed the scale of the central engine. Examples include the radio galaxy NGC 6251, where knots in the radio jets were inferred to accelerate from ~0.13c at a distance of ~0.53 pc from the galactic nucleus to ~0.42c at r=1.0 pc, and the quasar 3C 345, where the Lorentz factor of the radio knot C7 was deduced to increase from ~5 to >10 as it moved from r=3 pc to r=20 pc. It is argued, using exact semianalytic solutions of the relativistic MHD equations, that this behavior is a signature of magnetic acceleration. The same basic driving mechanism may apply to the relativistic jets in AGNs, gamma-ray burst sources, and microquasars.