Relativistic Poynting-Flux Jets as Transmission Lines

TL;DR

This paper models relativistic Poynting-flux jets as transmission lines with specific electrical properties, explaining observed phenomena like jet disturbances and localized radiation in active galactic nuclei.

Contribution

It develops a transmission line model for relativistic Poynting-flux jets, linking electromagnetic theory with astrophysical observations of jets in galaxies.

Findings

Calculated electromagnetic energy flow using observed currents.

Predicted disturbance propagation speeds and impedance in jets.

Explained localized dissipation and radiation in jets due to perturbations.

Abstract

Recent radio emission, polarization, and Faraday rotation maps of the radio jet of the galaxy 3C 303 have shown that one knot of this jet has a {\it galactic}-scale electric current of $\sim 3\times 10^{18}$ Amp\`ere flowing along the jet axis (Kronberg et al. 2011). We develop the theory of relativistic Poynting-flux jets which are modeled as a transmission line carrying a DC current $I_0$, having a potential drop $V_0$, and a definite impedance ${\cal Z}_0 =90(u_z/c)\Omega$, where $u_z$ is the bulk velocity of the jet plasma. The electromagnetic energy flow in the jet is ${\cal Z}_0 I_0^2$. The observed current in 3C 303 can be used to calculate the electromagnetic energy flow in this magnetically dominated jet. Time-dependent but not necessarily small perturbations of a Poynting-flux jet - possibly triggered by a gas cloud penetrating the jet - are described by "telegrapher's equations," which predict the propagation speed of disturbances and the effective wave impedance ${\cal Z}$. The disturbance of a Poynting jet by the cloud gives rise to localized dissipation in the jet which may explain the enhanced synchrotron radiation in the knots of the 3C 303 jet, and in the apparently stationary knot HST-1 in the jet from the nucleus of the galaxy M87 (Biretta et al. 1999).