A relativistic electron-positron beam propagating through a magnetized electron-ion plasma is shown to generate both circularly and linearly polarized synchrotron radiations, which is intrinsically linked with asymmetric energy dissipation of the pair beam during the filamentation instability dynamics in the background plasma. The ratio of both polarizations $|⟨P_{\mathrm{circ}}⟩/⟨P_{\mathrm{lin}}⟩|\sim 0.15$, occurring for a wide range of beam-plasma parameters, can help in understanding the recent observation of circularly polarized radiation from gamma-ray bursts.

• Received 17 December 2018
• Revised 26 March 2019

DOI:https://doi.org/10.1103/PhysRevLett.122.204801