Synchrotron polarization of a hybrid distribution of relativistic thermal and nonthermal electrons in GRB prompt emission

Abstract: Synchrotron polarization of relativistic nonthermal electrons in gamma-ray bursts (GRBs) has been widely studied. However, recent numerical simulations of relativistic shocks and magnetic reconnection have found that a more realistic electron distribution consists of a power-law component plus a thermal component, which requires observational validation. In this paper, we investigate synchrotron polarization using a hybrid energy distribution of relativistic thermal and nonthermal electrons within a globally toroidal magnetic field in GRB prompt emission. Our results show that, compared to the case of solely non-thermal electrons, the synchrotron polarization degrees (PDs) in these hybrid electrons can vary widely depending on different parameters and that the PD decreases progressively with frequency in the $\gamma$-ray, X-ray, and optical bands. The time-averaged PD spectrum displays a significant bump in the $\gamma$-ray and X-ray bands with the PDs higher than $\sim60\%$ if the thermal peak energy of electrons is much smaller than the conjunctive energy of electrons between the thermal and non-thermal distribution. The high synchrotron PD ($\gtrsim 60\%$) in the $\gamma$-ray and X-ray bands, which generally can not be produced by solely non-thermal electrons with typical power-law slopes, can be achieved by the hybrid electrons and primarily originates from the exponential decay part of the thermal component. Moreover, this model can roughly explain the PDs and spectral properties of some GRBs, where GRB 110301A with a high PD ($70_{-22}^{+22} \%$) may be potential evidence for the existence of relativistic thermal electrons.