The polarization of afterglow emission reveals γ-ray bursts jet structure

We numerically compute light and polarization curves of gamma-ray burst (GRB) afterglows for various configurations of the jet luminosity structure and for different dynamical evolutions. We especially consider the standard homogeneous 'top hat' jet and the 'universal structured jet' with power-law wings. We also investigate a possible, more physical variation of the 'top hat' model: the 'Gaussian jet'. The polarization curves for the last two jet types are shown here for the first time together with the computation of X-ray and radio polarized fluxes. We show that the light curves of the total flux from these configurations are very similar to each other, and therefore only very high quality data could allow us to pin down the underlying jet structure. We demonstrate instead that polarization curves are a powerful means to solve the jet structure, since the predicted behaviour of polarization and its position angle at times around the jet break are very different, if not opposite. We conclude that the afterglow polarization measurements provide clear footprints of any outflow energy distribution (unlike the light curves of the total flux) and the joint analysis of the total and polarized flux should reveal the jet structure of GRBs.