Magnetar Driven Bubbles and the Origin of Collimated Outflows from GRBs

TL;DR

This paper models how magnetar winds inflate bubbles that can become collimated, explaining the origin of jet-like outflows in gamma-ray bursts and related phenomena, depending on the magnetic energy ratio.

Contribution

It introduces a model linking magnetar wind magnetization to the asymmetry and collimation of outflows, providing a physical mechanism for GRB jet formation.

Findings

Higher magnetic to total energy ratio leads to collimated outflows.

Initial spherical expansion transitions to collimation over time.

Model explains observed GRB jet structures and supernova asymmetries.

Abstract

We model the interaction between the wind from a newly formed rapidly rotating magnetar and the surrounding progenitor. In the first few seconds after core collapse the magnetar inflates a bubble of plasma and magnetic fields behind the supernova shock, which expands asymmetrically because of the pinching effect of the toroidal magnetic field, as in PWNe, even if the host star is spherically symmetric. The degree of asymmetry depends on the ratio of the magnetic energy to the total energy in the bubble. We assume that the wind by newly formed magnetars inflating these bubbles is more magnetized than for PWNe. We show that for a magnetic to total power supplied by the central magnetar $\sim 0.1$ the bubble expands relatively spherically while for values greater than 0.3, most of the pressure in the bubble is exerted close to the rotation axis, driving a collimated outflow out through the host star. This can account for the collimation inferred from observations of long-duration gamma-ray bursts (GRBs). Given that the wind magnetization increases in time, we thus suggest that the magnetar-driven bubble initially expands relatively spherically (enhancing the energy of the associated supernova) while at late times it becomes progressivelymore collimated (producing the GRB). Similar processes may operate in more modestly rotating neutron stars to produce asymmetric supernovae and lower energy transients such as X-ray flashes.