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 Rγ , 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. Subject headings: gamma rays:bursts—gamma rays: observations—gamma rays: theory—plasmas— radiation mechanisms: non-thermal— radiation mechanism: thermal