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 broadband 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 R _γ, which is typically ≳10¹⁵ cm, independent of 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.