The polarisation of afterglow emission reveals GRB jet structure

We numerically compute light and polarization curves of γ-ray burst afterglows for various configurations of the jet structure and for different dynamical evolutions. We consider a standard homogeneous jet and a structured jet with power-law wings. We show that the lightcurves from these two 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 show instead that polarisation curves are a powerful mean to solve the jet structure, since the predicted behaviour of polarisation and its position angle at times around the jet break (tjet) are opposite. Polarisation from homogeneous jets has a minimum at tjet associated with a rotation by 90 ◦ of the position angle, while polarisation from a structured jet has a maximum at tjet and it is characterised by a constant position angle throughout the entire evolution. Thus a comparison of the predicted polarised flux curves with data should in principle reveals GRB jet structure.