Linear and Circular polarization in ultra-relativistic synchrotron sources - implications to GRB afterglows

TL;DR

This paper extends synchrotron polarization theory to relativistic jets, analyzing linear and circular polarization in GRB afterglows, and concludes that observed high circular polarization cannot originate from optically thin synchrotron emission.

Contribution

It develops a comprehensive model for polarization in relativistic jets, including various magnetic field and electron distribution configurations, and assesses their implications for GRB observations.

Findings

High circular polarization in GRB afterglows is unlikely from intrinsic synchrotron emission.

The polarization depends strongly on magnetic field geometry and electron pitch-angle distribution.

Observed polarization levels challenge the simple synchrotron emission models.

Abstract

Polarization measurements from relativistic outflows are a valuable tool to probe the geometry of the emission region and the microphysics of the particle distribution. Indeed, 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. Here we consider optically thin synchrotron emission and we extend the theory of circular polarization from a point source to an extended radially expanding relativistic jet. We present numerical estimates for both linear and circular polarization in such systems. We consider different configurations of the magnetic field, spherical and jetted outflows, isotropic and anisotropic pitch-angle distributions, and outline the difficulty in obtaining the reported high level of circular polarization observed in the afterglow of GRB 121024A. We conclude that the origin of the observed polarization cannot be intrinsic to an optically thin synchrotron process, even when the electron pitch-angle distribution is extremely anisotropic.