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 postshock magnetic field and structure, as well as the accelerated electron energy distribution. Here we show that X-ray afterglow observations on day timescales constrain the pre-shock magnetic field to satisfy B > 0.2(n/1 cm~(-3))~(5/8) mG, where n is the preshock density. This suggests that either the shock propagates into a highly magnetized fast, υ ～ 10~3 km s~(-1), wind, or the preshock 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 day, times.
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机译：伽马射线爆裂（GRB）余辉发射被认为是由相对论的无碰撞冲击传播到弱磁化等离子体后加速到高能量的电子的同步加速器发射而产生的。余辉观测已被用来约束震后磁场和结构以及加速的电子能量分布。在这里，我们显示了在日时间尺度上的X射线余辉观测值约束了震前磁场满足B> 0.2（n / 1 cm〜（-3））〜（5/8）mG，其中n是震前密度。这表明，或者是震动传播到高度磁化的快速υ〜10〜3 km s〜（-1），风中，或者是震前磁场被强烈放大，最有可能是高能震动加速粒子的流过。可以通过在大于1天的后期以高光子能量进行余辉观测来获得更严格的约束。
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