The structure of weakly-magnetized $ \gamma $-ray burst jets

TL;DR

This paper investigates how weak magnetic fields influence the structure and stability of gamma-ray burst jets, showing that even weak magnetization can suppress instabilities and alter jet morphology, especially in long GRBs.

Contribution

It demonstrates the stabilizing effect of weak toroidal magnetic fields on GRB jets and their impact on jet structure and mixing, extending previous hydrodynamic models.

Findings

Weak magnetic fields stabilize jets against local instabilities.

Magnetization reduces jet-cocoon interface and alters jet morphology.

Long GRB jets show more significant effects of magnetization than short GRB jets.

Abstract

The interaction of gamma-ray burst (GRB) jets with the dense media into which they are launched promote the growth of local hydrodynamic instabilities along the jet boundary. In a companion paper we study the evolution of hydrodynamic (unmagnetized) jets, finding that mixing of jet-cocoon material gives rise to an interface layer, termed jet-cocoon interface (JCI), which contains a significant fraction of the system energy. We find that the angular structure of the jet + JCI, when they reach the homologous phase, can be approximated by a flat core (the jet) + a power-law function (the JCI) with indices that depend on the degree of mixing. In this paper we examine the effect of subdominant toroidal magnetic fields on the jet evolution and morphology. We find that weak fields can stabilize the jet against local instabilities. The suppression of the mixing diminishes the JCI and thus reshapes the jet's post-breakout structure. Nevertheless, the overall shape of the outflow can still be approximated by a flat core + a power-law function, although the JCI power-law decay is steeper. The effect of weak fields is more prominent in long GRB jets, where the mixing in hydrodynamic jets is stronger. In short GRB jets there is small mixing in both weakly magnetized and unmagnetized jets. This result influences the expected jet emission which is governed by the jet's morphology. Therefore, prompt and afterglow observations in long GRBs may be used as probes for the magnetic nature at the base of the jets.