The propagation of relativistic jets in external media

TL;DR

This paper introduces an analytic model describing how relativistic jets interact with their surrounding medium, influencing their collimation and evolution across different astrophysical systems like AGNs, GRBs, and microquasars.

Contribution

The paper presents a self-consistent analytic model that predicts jet-cocoon dynamics and collimation based on key physical parameters, applicable across various astrophysical jet sources.

Findings

Jet-cocoon interaction depends on the ratio of jet energy density to ambient rest-mass energy density.

AGN and microquasar jets are hydrodynamically collimated by the ambient medium.

GRB jets are collimated inside stars but become uncollimated after breakout.

Abstract

Relativistic jets are ubiquitous in astrophysical systems that contain compact objects. They transport large amounts of energy to large distances from the source, and their interaction with the ambient medium has a crucial effect on the evolution of the system. The propagation of the jet is characterized by the formation of a shocked "head" at the front of the jet which dissipates the jet's energy and a cocoon that surrounds the jet and potentially collimates it. We present here a self consistent, analytic model that follows the evolution of the jet and its cocoon, and describes their interaction. We show that the critical parameter that determines the properties of the jet-cocoon system is the dimensionless ratio between the jet's energy density and the rest-mass energy density of the ambient medium. This parameter, together with the jet's injection angle, also determines whether the jet is collimated by the cocoon or not. The model is applicable to relativistic, unmagnetized, jets on all scales and may be used to determine the conditions in AGNs jets as well as in GRBs or microquasars. It shows that AGN and microquasar jets are hydrodynamically collimated due to the interaction with the ambient medium, while GRB jets can be collimated only inside a star and become uncollimated once they breakout.