Relativistic outflows appear in a wide variety of astrophysical sources, from Galactic micro-quasars to cosmological gamma-ray bursts (GRBs) and active galactic nuclei (AGN). In most cases the relativistic outflow is thought to arise from accretion onto a black hole, while neutron-stars are also known to produce relativistic outflows (either in the form of a steady wind or an impulsive ejection of ...