  Observations of gamma-ray bursts by the Fermi satellite, capable of detecting
photons in a very broad energy band: 8keV to >300GeV, have opened a new window
for the study of these enigmatic explosions. It is widely assumed that photons
of energy larger than 100 MeV are produced by the same source that generated
lower energy photons -- at least whenever the shape of the spectrum is a Band
function. We report here a surprising discovery -- the Fermi data for a bright
burst, GRB 080916C, unambiguously shows that the high energy photons (>=
100MeV) were generated in the external shock via the synchrotron process, and
the lower energy photons had a distinctly different source. The magnetic field
in the region where high energy photons were produced (and also the late time
afterglow emission region) is found to be consistent with shock compressed
magnetic field of the circum-stellar medium. This result sheds light on the
important question of the origin of magnetic fields required for gamma-ray
burst afterglows. The external shock model for high energy radiation makes a
firm prediction that can be tested with existing and future observations.
