FRII RGs: jet dynamics and episodic behaviour
  Radio galaxies are episodic in nature. In our recent work, our study of jet properties and their dynamics in different episodes of activity has revealed various hitherto unknown aspects of the extragalactic jets. We discover that the injection spectral indices are similar in the two different episodes for most of the episodic radio galaxies in our sample. We argue that in order to produce the similar injection indices in two episodes (i) the jet power in different episodes have to be similar and (ii) the Lorentz factor of the spine of the jet should be > 10. We further argue from particle acceleration physics that (iii) the jet fluid is made of electron-positron plasma and (iv) the inner jets of double-double radio galaxies are capable of forming hotspots even when propagating through the tenuous, nonthermal electron-positron plasma of the outer cocoon without any thermal matter in it. The episodic nature of the FR\,II radio galaxies we have studied appears to be unrelated to AGN feedback on the ambient medium. 
C. Konar, M.J. Hardcastle
astro-ph.HE