Efficient Acceleration of Relativistic Magnetohydrodynamic Jets

TL;DR

This paper investigates conditions for efficient and rapid energy conversion in relativistic MHD jets, highlighting the importance of external pressure profiles and magnetic field structures for matching observational data.

Contribution

It introduces new boundary condition considerations and demonstrates how external pressure profiles influence energy conversion efficiency in relativistic jets.

Findings

Rapid external pressure decrease enhances energy conversion

Specific poloidal magnetic field structures facilitate acceleration

Differences between magnetosonic and sonic points clarified

Abstract

Relativistic jets in active galactic nuclei, galactic microquasars, and gamma-ray bursts are widely considered to be magnetohydrodynamically driven by black hole accretion systems, although conversion mechanism from Poynting into particle kinetic energy flux is still open. Recent detailed numerical and analytical studies of global structures of steady, axisymmetric magnetohydrodynamic (MHD) flows with specific boundary conditions have not reproduced as rapid an energy conversion as required by observations. In order to find more suitable boundary conditions, we focus on the flow along a poloidal magnetic field line just inside the external boundary, without treating transfield force balance in detail. We find some examples of the poloidal field structure and corresponding external pressure profile for an efficient and rapid energy conversion as required by observations, and that the rapid acceleration requires a rapid decrease of the external pressure above the accretion disk. We also clarify the differences between the fast magnetosonic point of the MHD flow and the sonic point of de Laval nozzle.