Polarization From A Radially Stratified Off-Axis GRB Outflow

TL;DR

This paper models the polarization of synchrotron emission in off-axis gamma-ray burst outflows, providing insights into their angular structure and magnetic fields, especially for events like GRB 170817A.

Contribution

It introduces a theoretical polarization model for radially stratified off-axis GRB outflows in different environments, applied to observed bursts and GW-associated events.

Findings

Polarization depends on outflow structure and magnetic field geometry.

Constraints on magnetic fields in GRB 170817A from radio polarization limits.

Model explains polarization features in off-axis GRB afterglows.

Abstract

While the dominant radiation mechanism gamma-ray bursts (GRBs) remains a question of debate, synchrotron emission is one of the foremost candidates to describe the multi-wavelength afterglow observations. As such, it is expected that GRBs should present some degree of polarization across their evolution - presenting a feasible means of probing these bursts' energetic and angular properties. Although obtaining polarization data is difficult due to the inherent complexities regarding GRB observations, advances are being made, and theoretical modeling of synchrotron polarization is now more relevant than ever. In this manuscript, we present the polarization for a fiduciary model where the synchrotron forward-shock emission evolving in the radiative-adiabatic regime is described by a radially stratified off-axis outflow. This is parameterized with a power-law velocity distribution and decelerated in a constant-density and wind-like external environment. We apply this theoretical polarization model for selected bursts presenting evidence of off-axis afterglow emission, including the nearest orphan GRB candidates observed by the Neil Gehrels Swift Observatory and a few Gravitational Wave (GWs) events that could generate electromagnetic emission. In the case of GRB 170817A, we require the available polarimetric upper limits in radio wavelengths to constrain its magnetic field geometry.