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Long-lived >100 MeV emission has been a common feature of most Fermi-LATdetected gamma-ray bursts (GRBs), e.g., detected up to ~10^3s in long GRBs080916C and 090902B and ~10^2s in short GRB 090510. This emission is consistentwith being produced by synchrotron emission of electrons accelerated to highenergy by the relativistic collisionless shock propagating into the weaklymagnetized medium. Here we show that this high-energy afterglow emissionconstrains the preshock magnetic field to satisfy 1(n/1cc)^{9/8}mG<B<10^2(n/1cc)^{3/8}mG, where n is the preshock density, more stringent thanthe previous constraint by X-ray afterglow observations on day scale. Thissuggests that the preshock magnetic field is strongly amplified, most likely bythe streaming of high energy shock accelerated particles.

The upstream magnetic field of collisionless GRB shocks: constraint by Fermi-LAT observations