Linear and circular polarization in GRB afterglows

TL;DR

This paper models linear and circular polarization in GRB afterglows to interpret recent polarization measurements and understand the micro-physics of the emitting regions.

Contribution

It provides numerical estimates of polarization levels for various magnetic field, geometry, and electron distribution configurations, enhancing interpretation of observations.

Findings

Maximum and minimum polarization values identified for different scenarios

Conditions for high polarization levels established

Implications for magnetic field and electron distribution micro-physics discussed

Abstract

A certain degree of linear polarization has been measured in several GRB afterglows. More surprisingly, circular polarization has been recently measured in GRB121024A. For synchrotron emission, the polarization level depends on: (i) the local magnetic field orientation (ii) the geometry of the emitting region with respect to the line of sight and (iii) the electron pitch-angle distribution. For this reason, polarization measurements are a valuable tool to probe afterglow micro-physics. We present numerical estimates of linear and circular polarization for different configurations (i.e., magnetic fields, geometries and pitch-angle distributions). For each different scenario, we study the conditions for reaching the maximum and minimum linear and circular polarization and provide their values. We discuss the implication of our results to the micro-physics of GRB afterglows in view of recent polarization measurements.