Simulation of relativistic jets

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Abstract
Superluminal motion is often observed in extragalactic jets, but few simulations take into account the nature of relativistic fluids. Here, I have studied the behaviour of steady-state, supersonic jets propagating with a relativistic bulk velocity through a pressure gradient, similar to the observed atmospheres of extragalactic radio source hosts. The simulations were carried out in axisymmetry, using a Godunov-type fluid code in Lagrangian coordinates. The equation of state for a two-component ‘Synge gas’ was employed to close the system of conservation equations. It was found that initially hot jets (i.e. thermal energy of particles greater than rest energy) show large increases in bulk Lorentz factor during adiabatic cooling. In simulations where the jet experienced a period of significant over-pressure, and a resultant over-expansion, the familiar diamond-like structure of incident and reflected oblique shocks was seen. The final wavelength of the associated oscillations was revealed to be a function of initial Mach number and not temperature. It was also noted that, due to increased dissipation, the shock strength decays faster from one oscillation to the next as the initial temperature of the jet is raised.

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