A Powerful Hydrodynamic Booster for Relativistic Jets

Velocities close to the speed of light are a robust observational property ofthe jets observed in microquasars and AGNs, and are expected to be behind muchof the phenomenology of GRBs. Yet, the mechanism boosting relativistic jets tosuch large Lorentz factors is still essentially unknown. Building on recentgeneral-relativistic, multidimensional simulations of progenitors of shortGRBs, we discuss a new effect in relativistic hydrodynamics which can act as anefficient booster in jets. This effect is purely hydrodynamical and occurs whenlarge velocities tangential to a discontinuity are present in the flow,yielding Lorentz factors $\Gamma \sim 10^2-10^3$ or larger in flows withmoderate initial Lorentz factors. Although without a Newtonian counterpart,this effect can be explained easily through the most elementary hydrodynamicalflow: i.e., a relativistic Riemann problem.Comment: 4 pages, 4 figures (1 in color). ApJ Letters accepte