  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.
