Evidence of an initially magnetically dominated outflow in GRB 080916C

TL;DR

Analysis of GRB 080916C's broad band spectra indicates the outflow was initially dominated by magnetic energy, challenging the traditional baryonic fireball model and suggesting a Poynting flux component.

Contribution

This study provides evidence that the GRB outflow was initially magnetically dominated, a novel insight into GRB composition and emission mechanisms.

Findings

The prompt emission radius is >~ 10^{15} cm, independent of emission process.

No thermal component detected, contradicting baryonic models.

Poynting flux constitutes at least 15-20% of the total energy at the photosphere.

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.