The change of GRB polarization angles in the magnetic-dominated jet model

TL;DR

This paper explains the observed 90-degree change in GRB polarization angles during the prompt phase using a magnetic-dominated jet model where jet acceleration affects polarization.

Contribution

It demonstrates that polarization angle changes in GRBs can be naturally explained by the magnetic-dominated jet model with jet acceleration and electron-photon interactions.

Findings

Polarization angle changes are consistent with jet acceleration in the MDJ model.

Synchrotron photons' polarization is affected by Compton scattering at the jet front.

The model explains the 90-degree polarization angle shift observed in GRB 100826A.

Abstract

The polarimetric measurement on the prompt phase of GRB 100826A shows that the polarization angle changes $\sim 90^{\circ}$ between two adjacent time intervals. We will show that this phenomenon can be naturally interpreted in the framework of the magnetic-dominated jet (MDJ) model. The MDJ model suggests that the bulk Lorentz factor of the outflow increases as $\Gamma\propto r^{1/3}$, until reaching a saturated value $\Gamma_{\rm sat}$. Electrons move in the globally ordered magnetic field advected by the jet from the central engine and produce synchrotron photons. The polarized synchrotron photons travel alone the jet direction and then collide with the cold electrons at the front of the jet. After the Compton scattering process, these photons escape from the jet and are detected by the observer locating slightly off-axis. If photons are emitted before the bulk Lorentz factor saturates, the change of polarization angle is a natural result of the acceleration of the outflow.