Linear and circular polarization in ultra-relativistic synchrotron sources - implications to GRB afterglows
Date Issued
2016
Author(s)
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 Gamma Ray Burst (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.
Volume
455
Issue
2
Start page
1594
Issn Identifier
0035-8711
Ads BibCode
2016MNRAS.455.1594N
Rights
open.access
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