Afterglow Polarizations in a Stratified Medium with Effect of the Equal Arrival Time Surface

The environment of a & gamma;-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 & gamma; at the late stage. Through the influences on the f (32) or & gamma;, the observational energy band, observational angles, and the parameters of the stratified medium will finally affect the afterglow polarizations.

Automated summary

The influence of the equal arrival time surface (EATS) effect in a stratified medium on afterglow polarizations is investigated, and the polarization properties of multiband afterglows are predicted. The results show that the EATS effect becomes important for off-axis detections and causes the polarization degree (PD) bumps to shift to later times. The polarization properties of jet emission can evolve even with fixed magnetic field configurations, jet structure, and observational angles, and this evolution is mainly determined by the flux ratio f(32) and the bulk Lorentz factor γ.