A powerful hydrodynamic booster for relativistic jets

Velocities close to the speed of light are a robust observational property of the jets observed in microquasars and active galactic nuclei, and they are expected to be behind much of the phenomenology of gamma-ray bursts (GRBs). Yet the mechanism boosting relativistic jets to such large Lorentz factors is not fully known. Building on recent general relativistic, multidimensional simulations of progenitors of short GRBs, we discuss a new effect in relativistic hydrodynamics that can act as an efficient booster in jets. This effect is purely hydrodynamical and occurs when large velocities tangential to a discontinuity are present in the flow, yielding Lorentz factors Gamma similar to 10(2)-10(3) or larger in flows with moderate initial Lorentz factors. Although without a Newtonian counterpart, this effect can be explained easily through the most elementary hydrodynamical flow (i.e., a relativistic Riemann problem).