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

Lara Nava, Ehud Nakar, Tsvi Piran

Research output: Contribution to journalArticlepeer-review


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.

Original languageEnglish
Pages (from-to)1594-1606
Number of pages13
JournalMonthly Notices of the Royal Astronomical Society: Letters
Issue number2
StatePublished - 1 Jan 2020


  • Radiation mechanisms: non-thermal

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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