Dynamical efficiency of collisionless magnetized shocks in relativistic jets

Abstract

The so‐called internal shock model aims to explain the light‐curves and spectra produced by non‐thermal processes originated in the flow of blazars and gamma‐ray bursts. A long standing question is whether the tenuous collisionless shocks, driven inside a relativistic flow, are efficient enough to explain the amount of energy observed as compared with the expected kinetic power of the outflow. In this work we study the dynamic efficiency of conversion of kinetic‐to‐thermal/magnetic energy of internal shocks in relativistic magnetized outflows. We find that the collision between shells with a non‐zero relative velocity can yield either two oppositely moving shocks (in the frame where the contact surface is at rest), or a reverse shock and a forward rarefaction. For moderately magnetized shocks (magnetization σ ≃ 0.1), the dynamic efficiency in a single two‐shell interaction can be as large as 40%. Hence, the dynamic efficiency of moderately magnetized shocks is larger than in the corresponding unmagnetized two‐shell interaction. We find that the efficiency is only weakly dependent on the Lorentz factor of the shells and, thus internal shocks in the magnetized flow of blazars and gamma‐ray bursts are approximately equally efficient.