Jets from Scratch: A 3D Dynamo Origin of Long Gamma-Ray Burst Jets

TL;DR

This paper presents 3D relativistic magnetohydrodynamic simulations showing how in situ dynamo processes can generate poloidal magnetic fields from toroidal fields in collapsars, powering variable jets akin to long gamma-ray bursts.

Contribution

First 3D GRMHD simulations demonstrating in situ dynamo-driven poloidal field generation in collapsars, leading to variable jet formation without inherited magnetic fields.

Findings

Dynamo-generated poloidal fields seed relativistic jets.

Jets exhibit variability and misalignment with black hole spin.

Magnetic flux inversions produce striped jets potentially observable in LGRB light curves.

Abstract

The origin of the large-scale poloidal magnetic field required to power relativistic jets in collapsars remains uncertain. While such a field may be inherited during PNS collapse, the efficiency of this process is unclear, motivating an in situ mechanism to generate poloidal fields out of the predominantly toroidal fields produced by stellar differential rotation. We present the first 3D general-relativistic magnetohydrodynamic collapsar simulations initialized with toroidal magnetic field profiles that closely follows those of pre-collapse stellar models. As the toroidal field in the disk becomes dynamically important, it seeds the dynamo, producing coherent poloidal magnetic loops that appear at $\sim \mathcal{O}(100)$ gravitational radii and are then advected inward along paths that may deviate from the disk midplane. The resulting poloidal fields thread the black hole (BH) and launch highly variable, wobbling relativistic jets on timescales of order seconds, with the onset depending on the initial magnetic field and the plasma circularization radius. Although the jets are highly variable and misaligned with the BH spin axis, they sustain $\gtrsim 10^{50}$ erg s$^{-1}$, comparable to that inferred for long gamma-ray bursts (LGRB). We identify magnetic-flux inversions driven by the stochastic dynamo, leading to the formation of striped jets that could be imprinted in LGRB light curves. These results demonstrate that the accretion0disk dynamo provides a robust pathway for jet production in collapsars across a broad range of progenitors.