Transformation of the Poynting flux into kinetic energy in relativistic jets

Abstract

The acceleration of relativistic jets from the Poynting-to the matter-dominated stage is considered. There are generally two collimation regimes, which we call equilibrium and non-equilibrium respectively. In the first regime, the jet is efficiently accelerated up to the equipartition between the kinetic and electromagnetic energy. We show that after equilibrium the jet ceases to be Poynting-dominated and the ratio of electromagnetic to kinetic energy decreases only logarithmically, so that such jets become truly matter-dominated only at extremely large distances. Non-equilibrium jets remain generally Poynting-dominated until logarithmically large distances. In the only case when a non-equilibrium jet is accelerated to the equipartition level, we found that the flow is not continued to infinity but is focused towards the axis at a finite distance from the origin.