Collimation and confinement of magnetic jets by external media

TL;DR

This paper models how external media confine magnetic jets, leading to instabilities that may convert magnetic energy into thermal energy, with applications to gamma-ray burst jets and their observed emissions.

Contribution

It introduces a self-consistent model of jet-cocoon evolution considering magnetic instabilities and applies it to GRB jet breakout scenarios.

Findings

Jets are highly collimated inside progenitor stars.

Magnetic instabilities likely cause magnetic field destruction.

Confined jets may produce low-sigma outflows responsible for GRB emission.

Abstract

We study the collimation of a highly magnetized jet by a surrounding cocoon that forms as a result of the interaction of the jet with the external medium. We show that in regions where the jet is well confined by the cocoon, current-driven instabilities should develop over timescales shorter than the expansion time of the jet's head. We speculate that these instabilities would give rise to complete magnetic field destruction, whereby the jet undergoes a transition from high-to-low sigma above the collimation zone. Using this assumption, we construct a self-consistent model for the evolution of the jet-cocoon system in an ambient medium of arbitrary density profile. We apply the model to jet breakout in long GRBs, and show that the jet is highly collimated inside the envelope of the progenitor star, and is likely to remain confined well after breakout. We speculate that this strong confinement may provide a channel for magnetic field conversion in GRB outflows, whereby the hot, low-sigma jet section thereby produced is the source of the photospheric emission observed in many bursts.