Relativistic AGN jets II. Jet properties and mixing effects for episodic
  jet activity
  Various radio galaxies show signs of having gone through episodic jet
outbursts in the past. An example is the class of double-double radio galaxies
(DDRGs). However, to follow the evolution of an individual source in real-time
is impossible due to the large time scales involved. Numerical studies provide
a powerful tool to investigate the temporal behavior of episodic jet outbursts
in a (magneto-)hydrodynamical setting. We simulate the injection of two jets
from active galactic nuclei (AGN), separated by a short interruption time.
Three different jet models are compared. We find that an AGN jet outburst cycle
can be divided into four phases. The most prominent phase occurs when the
restarted jet is propagating completely inside the hot and inflated cocoon left
behind by the initial jet. In that case, the jet-head advance speed of the
restarted jet is significantly higher than the initial jet-head. While the head
of the initial jet interacts strongly with the ambient medium, the restarted
jet propagates almost unimpeded. As a result, the restarted jet maintains a
strong radial integrity. Just a very small fraction of the amount of shocked
jet material flows back through the cocoon compared to that of the initial jet
and much weaker shocks are found at the head of the restarted jet. We find that
the features of the restarted jet in this phase closely resemble the observed
properties of a typical DDRG.
