Afterglow light curves from magnetized GRB flows

Using the RMHD code MRGENESIS and the radiative transfer code SPEV we compute multiwavelength afterglow light curves of magnetized ejecta of gamma-ray bursts interacting with a uniform circumburst medium. We are interested in the emission from the reverse shock when ejecta magnetization varies from σ₀ = 0 to σ₀ = 1. For typical parameters of the ejecta, the emission from the reverse shock peaks for magnetization σ₀ ~ 0.01 - 0.1, and is suppressed for higher σ₀. We fit the early afterglow light curves of GRB 990123 and 090102 and discuss the possible magnetization of the outflows of these bursts. Finally we discuss the amount energy left in the magnetic field which is available for dissipation at later afterglow stages.