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(Abridged) We aim to quantitatively understand the dynamical effect andobservational signatures of magnetization of the GRB ejecta on the onset of theafterglow. We perform ultrahigh-resolution one-dimensional relativistic MHDsimulations of the interaction of a radially expanding, magnetized ejecta withthe interstellar medium. The need of ultrahigh numerical resolution derivesfrom the extreme jump conditions in the region of interaction between theejecta and the circumburst medium. We study the evolution of anultrarelativistic shell all the way to a the self-similar asymptotic phase. Oursimulations show that the complete evolution can be characterized in terms oftwo parameters, namely, the \xi parameter introduced by Sari & Piran (1995) andthe magnetization \sigma_0. We exploit this property by producing numericalmodels where the shell Lorentz factor is \gamma_0 ~ tens and rescaling theresults to arbitrarily large \gamma_0. We find that the reverse shock istypically very weak or absent for ejecta characterized by \sigma_0 >~ 1. Theonset of the forward shock emission is strongly affected by the magnetization.On the other hand, the magnetic energy of the shell is transfered to theexternal medium on a short timescale (~several times the duration of theburst). The later forward shock emission does not contain information for theinitial magnetization of the flow. The asymptotic evolution of stronglymagnetized shells, after they have suffred a substantial deceleration,resembles that of hydrodynamic shells, i.e., they fully enter in theBlandford-McKee self-similar regime.

Deceleration of arbitrarily magnetized GRB ejecta: the complete evolution