Particle acceleration and dynamics of double-double radio galaxies:
  theory vs. observations
  In this paper we show that a small sample of radio galaxies with evidence for
multiple epochs of jet activity (so-called `double-double' radio galaxies) have
the same electron injection spectral index in the two activity episodes, a
result which might be considered surprising given the very different lobe
dynamics expected in the first and second episode. We construct models for the
dynamics of radio galaxies, with an emphasis on their episodic behaviour, and
show that hotspot formation and confinement of lobes for the inner double of
double-double radio galaxies are possible even without any thermal matter in
the outer cocoon. We argue that (i) the observed similar injection spectral
indices are due to similar jet powers in the two episodes, (ii) the `spectral
index--radio power' correlation of a flux limited sample of radio galaxies is
the primary one, and not the `spectral index--redshift correlation', (iii) jets
are made of pair plasma and not electron-proton, (iv) and the Lorentz factor of
the spine of the jet should be $\gapp 10$ to explain the observations.
Furthermore, we argue that the observations show that higher power radio
galaxies do not have a higher jet bulk Lorentz factors, but instead simply have
a higher number density of particles in the jet rest frame. A consequence of
our models is that aligned double-double radio galaxies with very old
($\gapp10^8$ yr) outer doubles, or misaligned double-double radio galaxies, are
statistically more likely to have dissimilar injection indices in two different
episodes, as they will probably have different jet powers.
