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The upstream magnetic field of GRB shocks

Gamma-ray burst (GRB) afterglow emission is believed to be produced by synchrotron emission of electrons accelerated to high energy by a relativistic collisionless shock propagating into a weakly magnetized plasma. Afterglow observations have been used to constrain the post-shock magnetic field and structure, as well as the accelerated electron energy distribution. Here we show that X-ray afterglow observations on day time scale constrain the pre-shock magnetic field to satisfy B>0.2[n/(1/cc)]^{5/8} mG, where n is the pre-shock density. This suggests that either the shock propagates into a highly magnetized fast, v~10^3 km/s, wind, or that the pre-shock magnetic field is strongly amplified, most likely by the streaming of high energy shock accelerated particles. More stringent constraints may be obtained by afterglow observations at high photon energy at late, >1 d, times.