Exploring the Early Afterglow Polarization of GRB 190829A

TL;DR

This paper models the early afterglow polarization of GRB 190829A to understand magnetic field configurations and emission geometry, using polarization data to distinguish between on-axis and off-axis scenarios.

Contribution

It introduces a combined polarization and synchrotron forward-shock model to interpret polarization evolution in GRB afterglows, addressing degeneracies in existing models.

Findings

On-axis emission with anisotropic magnetic fields is favored.

Off-axis scenarios remain plausible with isotropic magnetic fields.

Polarization data constrains magnetic field geometry and emission angle.

Abstract

The GRB 190829A has been widely studied due to its nature and the high energy emission presented. Due to the detection of a very-high-energy component by the High Energy Stereoscopic System and the event's atypically middling luminosity, it has been categorized in a select, limited group of bursts bordering classic GRBs and nearby sub-energetic events. Given the range of models utilized to adequately characterize the afterglow of this burst, it has proven challenging to identify the most probable explanation. Nevertheless, the detection of polarization data provided by the MASTER collaboration has added a new aspect to GRB 190829A that permits us to attempt to explore this degeneracy. In this paper, we present a polarization model coupled with a synchrotron forward-shock model -- a component in all models used to describe GRB 190829A's afterglow -- in order to fit the polarization's temporal evolution with the existing upper limits ($\Pi < 6\%$). We find that the polarization generated from an on-axis emission is favored for strongly anisotropic magnetic field ratios, while an off-axis scenario cannot be fully ruled out when a more isotropic framework is taken into account.