Origin of the twice ${90}^{\circ}$ rotations of the polarization angle in GRB 170114A and GRB 160821A

TL;DR

This paper explains the twice ${90}^{\u00b0}$ polarization angle rotations in certain gamma-ray bursts using a multi-shell model with aligned magnetic fields, favoring a magnetar central engine.

Contribution

It introduces a multi-shell model with different magnetic field configurations to interpret polarization rotations in GRBs, favoring aligned fields and magnetar engines.

Findings

Aligned magnetic fields explain the twice ${90}^{\u00b0}$ PA rotations.

Toroidal magnetic fields cannot account for the observed PA rotations.

Magnetar central engines are favored for the studied GRBs.

Abstract

The observed twice abrupt ${90}^{\circ}$ rotations of the polarization angle (PA) in the prompt phase of gamma-ray bursts (GRBs) are difficult to be understandable within the current one-emitting-shell models. Here, we apply a model with multiple emitting shells to solve this new challenging problem. Two configurations of large-scale ordered magnetic fields in the shells are considered: toroidal and aligned. Together with the light curves and the spectral peak-energy evolutions, the twice ${90}^{\circ}$ PA rotations in GRB 170114A and GRB 160821A could be well interpreted with the multi-shell aligned magnetic fields configuration. Our numerical calculations also show that the multiple shells with the toroidal magnetic field configuration could not explain the observed twice ${90}^{\circ}$ PA rotations. An aligned magnetic field configuration in the GRB outflow usually indicate to prefer a magnetar central engine, while a toroidal field configuration is typically related to a central black hole. Therefore, the magnetar central engines for the two GRBs are favored.