EVIDENCE OF AN INITIALLY MAGNETICALLY DOMINATED OUTFLOW IN GRB 080916C

Abstract

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