Constraints on the Nature of Jets from kpc Scale X-ray Data
  Motivated by the large number of jets detected by the Chandra X-ray Observatory, and by the inverse Compton X-ray emission model (IC/CMB) for relativistic jets, we revisit two basic questions: ``If the medium that carries the jet's energy consists of hot electrons, can we use the physical length of the jet to constrain the maximum electron energy?'' and ``Why do jets have knots?'' Based on the two non-thermal emission processes for X-rays from jets, we consider constraints on the jet medium and other properties from these two simple questions. We argue that hot pairs cannot be the dominant constituent of the medium responsible for the jet's momentum flux and that some mechanisms for producing fluctuating brightness along jets (rather than a monotonically decreasing intensity) are precluded by observed jet morphologies. 
D. E. Harris (SAO) and H. Krawczynski (Washington University in St.   Louis)