Comment on "Gamma-ray burst early afterglows: reverse shock emission from an arbitrarily magnetized ejecta" by Zhang and Kobayashi (2004)

Zhang and Kobayashi (2004) attempted to calculate early afterglow emission from a system of forward and reverse shocks in GRB outflows for the case of magnetized ejecta. We point out a fundamental error in the underlying dynamical model. According to the authors, energy and momentum carried by the magnetic field of the ejecta are not transfered to the forward shock. This is an incorrect assumption that invalidates the results.