Suprathermality effects on the magnetic equipartition parameter in GRB shocks

It has been widely accepted that the Weibel shock theory of Gamma-ray Bursts (GRBs) plays an important role in the amplification of the generated magnetic fields in space plasmas with non-thermal particle distributions. In baryon-dominated shocks, after saturation of the electron Weibel instability, the plasma is described by the waterbag and the Kappa power-law models for two counter-streaming proton beams and the background electrons, respectively. Based on a fully kinetic treatment, the interplay of κ-index on the equipartition magnetic field εB is unveiled. Two approaches; namely, (A) κ-independent and (B) κ-dependent temperatures are invoked here to compare the effects of the background suprathermal populations on the εB parameter. It is found that the maximum magnetic equipartition parameter (εB)max in model A (B) is reduced (enhanced) by the increase of suprathermal populations, i.e. decreasing κ-index. Furthermore, for a fixed value of the power-index κ, the amount of (εB)max predicted by the method B is higher than that predicted by the method A. Interestingly, within the framework of theoretical models, our findings align with εB domain experimentally reported for several GRB events.