Evidence of an initially magnetically dominated outflow in GRB 080916C
Abstract
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.
Keywords
Cite
@article{arxiv.0904.2943,
title = {Evidence of an initially magnetically dominated outflow in GRB 080916C},
author = {Bing Zhang and Asaf Pe'er},
journal= {arXiv preprint arXiv:0904.2943},
year = {2011}
}
Comments
ApJL, updated with the published version. Minor changes. References are limited to 50 according to the new ApJL rule