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Evidence of an initially magnetically dominated outflow in GRB 080916C

The composition of gamma-ray burst (GRB) ejecta is still a mystery. The standard model invokes an initially hot ``fireball'' composed of baryonic matter. Here we analyze the broad band spectra of GRB 080916C detected by the Fermi satellite. The featureless Band-spectrum of all five epochs as well as the detections of >~10 GeV photons in this burst place a strong constraint on the prompt emission radius, which is typically >~ 10^{15} cm, independent on the details of the emission process. The lack of detection of a thermal component as predicted by the baryonic models strongly suggests that a significant fraction of the outflow energy is initially not in the ``fireball'' form, but is likely in a Poynting flux entrained with the baryonic matter. The ratio between the Poynting and baryonic fluxes is at least ~(15-20) at the photosphere radius, if the Poynting flux is not directly converted to kinetic energy below the photosphere.