Double-Peaked Optical Afterglow in GRB 110213A Inferring a Magnetized Thick Shell Ejecta

TL;DR

This paper models the double-peaked optical afterglow of GRB 110213A using a magnetized thick shell ejecta framework, explaining the observed features and shallow X-ray decay through combined forward and reverse shock emissions.

Contribution

It introduces a semi-analytic model for magnetized, finite-thickness ejecta interacting with a stratified medium, explaining complex afterglow features of GRB 110213A.

Findings

Thick shell ejecta explains shallow X-ray decay.

Magnetized reverse shock causes optical double peaks.

Low radiative efficiency linked to high jet magnetization.

Abstract

Gamma-ray bursts early afterglows are important tracers for determining the radial structure and magnetization of the ejecta. In this paper, we focus on GRB 110213A that shows double-peaked optical afterglow lightcurves and the shallow decay feature of the X-ray afterglow. We adopt a semi-analytic model for the dynamics of forward and reverse shocks generated through an interaction between an arbitrary magnetized ejecta with a finite thickness and a stratified circumstellar medium. Multiwavelength radiation from forward and reverse shocks seen from an arbitrary viewing angle is calculated under a thin-shell approximation. Our analysis with multimodal nested sampling methods for GRB 110213A suggests that the thick shell ejecta naturally explains the shallow decay feature of the X-ray afterglow. The combination of the reverse shock emission in the strongly magnetized jet and forward shock emission in the weakly magnetized circumstellar medium makes the double peak feature of the optical afterglows. The estimated low radiative efficiency in the prompt phase may be a consequence of the high magnetization of the jet in this case. A multi-messenger emission simulator based on the magnetic bullet afterglow model is publicly available as the open source Julia package "Magglow".