Understanding the Puzzling Acceleration of Jets of Active Galactic Nuclei

As relativistic plasma launches from a compact object at the center of a galaxy, the corresponding outflow should slow down with the increase of separation from the core due to energy dissipation along the path. However, some long-baseline observations of active galactic nucleus (AGN) jets show that the velocity of jets increases rather than decreases at a larger and larger separation from the core. The mechanism of such an acceleration of astrophysical jets has not been well understood so far, although much progress has been achieved on theoretical and observational perspectives. This paper illustrates the phenomenon of jet acceleration that emerged in some AGNs by the nonballistic model in which some nonconsecutive knots are produced by a continuous outflow at different distances from the central black hole; such knots appear to rotate along different radii at the same precession cone in the case of a precessing jet. The projection of the trajectories of such knots on the plane of the sky leads us to expect that jet components further from the core move at larger apparent velocities. The investigation provides a very simple scenario to the puzzling phenomena of astrophysical jets.