2006ApJ...651..328L

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 preshock magnetic field to satisfy B>0.2(n/1/cm3)^5/8 mG, where n is the preshock density. This suggests that either the shock propagates into a highly magnetized fast, v∼10³ km/s, 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.