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The observed abrupt twice-90° rotations of the polarization angle (PA) in the prompt phase of gamma-ray bursts (GRBs) are difficult to understand 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° PA rotations in GRB 170114A and GRB 160821A could be well interpreted with the multishell aligned magnetic field configuration (MFC). Our numerical calculations also show that the multiple shells with the toroidal MFC could not explain the observed twice-90° PA rotations. An aligned MFC in the GRB outflow usually indicates the preference of 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.

The time-integrated polarization degree (PD) in the prompt optical band of a gamma-ray burst (GRB) was predicted to be less than 20%, while the time-resolved one can reach as high as 75% in the photosphere model. Polarizations in the optical band during the GRB prompt phase have not previously been studied in the framework of the magnetic reconnection model. Here, a three-segment power law of the energy spectrum is used to reconstruct the Stokes parameters of the magnetic reconnection model. Multiwavelength light curves and polarization curves from the optical band to MeV gamma rays in the GRB prompt phase are studied. We found that, depending mainly on the jet dynamics, there is a long-lasting high-PD phase in all calculated energy bands for the typical parameter sets. The time-resolved PD could be as high as 50%, while the time-integrated one is roughly 17% in the optical band. The time-resolved PD in X-rays can reach 60% and the time-integrated one is around 30%–40%. The evolution of polarization angle (PA) is random in both optical and gamma-ray bands for the photosphere model, while it is roughly constant in the synchrotron models. Therefore, future time-resolved PA observations in the prompt optical or gamma-ray band could distinguish between the photosphere and synchrotron models.

The environment of a γ -ray burst has an important influence on the evolution of jet dynamics and its afterglow. Here we investigate the afterglow polarizations in a stratified medium with the equal arrival time surface (EATS) effect. Polarizations of multiband afterglows are predicted. The effects of the parameters of the stratified medium on the afterglow polarizations are also investigated. We found the influences of the EATS effect on the afterglow polarizations become important for off-axis detections and polarization degree (PD) bumps move to later times with the EATS effect. Even if the magnetic field configurations, jet structure, and observational angles are fixed, polarization properties of the jet emission could still evolve. Here, we assume a large-scale ordered magnetic field in the reverse-shock region and a two-dimensional random field in the forward-shock region. Then, PD evolution is mainly determined by the evolution of the f _32 parameter (the flux ratio between the reverse-shock region and forward-shock region) at the early stage and by the evolution of the bulk Lorentz factor γ at the late stage. Through the influences on the f _32 or γ , the observational energy band, observational angles, and the parameters of the stratified medium will finally affect the afterglow polarizations.

The rotations of the polarization angle (PA) with time (energy) can lead to the depolarization of the time-integrated (energy-integrated) polarization. However, we do not know how and when it will rotate. Here, we consider a magnetic reconnection model to investigate the polarizations, especially the PA rotations of gamma-ray burst (GRB) prompt emission. For a large-scale ordered aligned magnetic field configuration, we find that within T _50 PAs will rotate with time (energy) for slight off-axis observations. Our studies show that the rotations of the PAs are due to changes in the observed image of the emitting region before being averaged over the equal arrival time surface for the unresolved sources. We apply our model to the single-pulse bursts of GRB 170101A and GRB 170114A with time-resolved PA observations. We find it can interpret the violent PA variation of GRB 170101A, while the model could not predict the two 90° PA changes in GRB 170114A. A more detailed model should be considered.