Synchrotron Polarization Radiative Transfer: Relativistic Thermal Electrons Contribution

TL;DR

This paper models synchrotron polarization from relativistic thermal electrons, incorporating radiative transfer effects, to interpret polarization observations in various astrophysical phenomena, suggesting a neutron star origin for FRBs.

Contribution

It introduces a hybrid thermal-nonthermal electron model for calculating synchrotron polarization, including radiative transfer effects, applied to diverse astrophysical sources.

Findings

Thermal electrons significantly influence polarization degrees.

Strong magnetic fields are required to match observed FRB polarization.

Radiative transfer effects reduce polarization in heavily absorbed media.

Abstract

Relativistic thermal electrons moving in a large-scale magnetic field can produce synchrotron radiation. Linear synchrotron polarization can also be produced by the relativistic thermal electrons. In this paper, we utilize a hybrid thermal-nonthermal electron energy distribution to calculate circular synchrotron polarization. We further compute the radiative transfer of the synchrotron polarization in the optical and radio bands when we consider the contribution of the thermal electrons. We attempt to apply the polarization results to some astrophysical objects, such as kilonova like AT 2017gfo/GW170817, the fast radio burst (FRB), the Gamma-ray burst (GRB) afterglow, and the supernova remnant (SNR). The large optical depth of radiative transfer effects the small polarization degrees of these populations when the media surrounding the synchrotron sources take heavy absorption to the polarized photons. We need a strong magnetic field in our model to reproduce the linear and circular polarization properties that were observed in FRB 140514. This indicates that FRBs have a neutron star origin.