Magnetohydrodynamic Effects in Propagating Relativistic Jets: Reverse Shock and Magnetic Acceleration

TL;DR

This paper investigates how magnetic fields influence the deceleration and acceleration of relativistic jets, revealing a transition from shocks to rarefaction waves that significantly accelerates the flow, with implications for gamma-ray bursts and active galactic nuclei.

Contribution

It provides a detailed analysis of the Riemann problem for magnetized relativistic flows, identifying the conditions under which shocks weaken or transition to rarefaction waves, leading to magnetic acceleration.

Findings

Reverse shock weakens with increasing magnetization   

Flow accelerates significantly in the rarefaction wave regime due to magnetic pressure

Critical magnetization    determines shock-to-rarefaction transition

Abstract

We solve the Riemann problem for the deceleration of an arbitrarily magnetized relativistic flow injected into a static unmagnetized medium in one dimension. We find that for the same initial Lorentz factor, the reverse shock becomes progressively weaker with increasing magnetization \sigma (the Poynting-to kinetic energy flux ratio), and the shock becomes a rarefaction wave when \sigma exceeds a critical value, \sigma_c, defined by the balance between the magnetic pressure in the flow and the thermal pressure in the forward shock. In the rarefaction wave regime, we find that the rarefied region is accelerated to a Lorentz factor that is significantly larger than the initial value. This acceleration mechanism is due to the strong magnetic pressure in the flow. We discuss the implications of these results for models of gamma-ray bursts and active galactic nuclei.