2006ApJ...651L...9M

The shock model of gamma-ray bursts (GRBs) contains two equipartition parameters: the magnetic energy density and the kinetic energy density of the electrons relative to the total energy density of the shock, ε_B and ε_e, respectively. These are free parameters within the model. Whereas the Weibel shock theory and numerical simulations fix ε_B at the level of a few times 10^–3 to 10^–4, no understanding of ε_e exists so far. Here we demonstrate that it inevitably follows from the theory that ε_e≃ε_B^1/2. The GRB afterglow data fully agree with this theoretical prediction. Our result explains why the electrons are close to equipartition in GRBs. The ε_e-ε_Brelation can potentially be used to reduce the number of free parameters in afterglow models.