On the generation of high energy photons detected by the Fermi Satellite from gamma-ray bursts

TL;DR

Fermi satellite observations of gamma-ray burst GRB 080916C reveal that high energy photons are produced by external shocks via synchrotron radiation, differing from lower energy photon sources, providing new insights into magnetic field origins.

Contribution

This study demonstrates that high energy photons in a gamma-ray burst originate from external shocks, challenging previous assumptions of a common source for all photon energies.

Findings

High energy photons (>= 100MeV) are produced in external shocks via synchrotron emission.

The magnetic field in the emission region is consistent with shock-compressed circum-stellar medium.

The external shock model predicts observable signatures for future gamma-ray burst observations.

Abstract

Observations of gamma-ray bursts by the Fermi satellite, capable of detecting photons in a very broad energy band: 8keV to >300GeV, have opened a new window for the study of these enigmatic explosions. It is widely assumed that photons of energy larger than 100 MeV are produced by the same source that generated lower energy photons -- at least whenever the shape of the spectrum is a Band function. We report here a surprising discovery -- the Fermi data for a bright burst, GRB 080916C, unambiguously shows that the high energy photons (>= 100MeV) were generated in the external shock via the synchrotron process, and the lower energy photons had a distinctly different source. The magnetic field in the region where high energy photons were produced (and also the late time afterglow emission region) is found to be consistent with shock compressed magnetic field of the circum-stellar medium. This result sheds light on the important question of the origin of magnetic fields required for gamma-ray burst afterglows. The external shock model for high energy radiation makes a firm prediction that can be tested with existing and future observations.