The steady-state structure of relativistic magnetic jets

Abstract

The method of characteristics is used to study the structure of steady, relativistic jets containing a toroidal magnetic field component. We assume axisymmetry and perfect conductivity for the fluid flows. Oblique, relativistic, magnetic shocks are handled using a shock-fitting procedure. The effects of the magnetic field on the collimation and propagation of the jets are studied when the external medium has a constant or decreasing pressure distribution. Our parameter study is confined to underexpanded jet flows which have an ultrarelativistic equation of state and extremely super-magnetosonic bulk velocities. The magnetic energy density, however, may range from zero to extreme dominance. These simulations are therefore relevant to compact radio jet sources which exhibit superluminal motion. For slightly underexpanded jets propagating into a constant-pressure external medium, the jet structure is quite periodic. This periodicity is enhanced as the toroidal field strength increases and the jet is strongly pinched. Recollimation occurs whether or not a toroidal field is present. When the jet propagates into an external medium of decreasing pressure its structure is very dependent upon the pressure gradient. For a pressure law p ∝ z–2, where z is the distance from the jet source, the periodic structure is lost. A nonmagnetic jet expands freely into the external medium and eventually comes into pressure equilibrium with it. A toroidal magnetic field cannot stop the jet from expanding. When p ∝ z–1, a semi-periodic structure is regained, and again this periodicity is particularly noticeable when a strong toroidal field is present; however, the magnetic field cannot prevent the jet from expanding, although it can greatly reduce the rate at which it does so.