Gamma-ray polarization induced by cold electrons via Compton processes

TL;DR

This paper investigates how Compton scattering of synchrotron photons by cold electrons in gamma-ray burst outflows affects observed polarization, explaining energy-dependent polarization and polarization angle changes.

Contribution

It introduces a model combining synchrotron radiation and Compton scattering to explain polarization features in GRBs, including angle shifts.

Findings

High-energy photons (>10 MeV) have lower polarization than low-energy photons (<1 MeV).

Maximum polarization occurs at specific viewing angles (~1/Γ).

The model explains the 90° polarization angle change in GRB 100826A.

Abstract

The polarization measurement is an important tool to probe the prompt emission mechanism in gamma-ray bursts (GRBs). The synchrotron photons can be scattered by cold electrons in the outflow via Compton scattering processes. The observed polarization depends on both the photon energy and the viewing angle. With the typical bulk Lorentz factor $\Gamma \sim 200$, photons with energy $E>10$ MeV tend to have smaller polarization than photons with energy $E<1$ MeV. At the right viewing angle, i.e. $\theta \sim \Gamma^{-1}$, the polarization achieves its maximal value, and the polarization angle changes $90^{\circ}$ relative to the initial polarization direction. Thus, the synchrotron radiation plus Compton scattering model can naturally explain the $90^{\circ}$ change of the polarization angle in GRB 100826A.