GRB 080916C: on the radiation origin of the prompt emission from KeV/MeV to GeV

TL;DR

This paper analyzes Fermi gamma-ray observations of GRB 080916C, demonstrating that its broad-spectrum emission from keV/MeV to GeV energies is consistent with synchrotron radiation from shock-accelerated electrons, and constrains shock parameters.

Contribution

It provides evidence supporting synchrotron origin of the prompt emission across a wide energy range and constrains shock microphysics and magnetic fields in GRB 080916C.

Findings

Spectrum consistent with synchrotron emission from shock-accelerated electrons.

High-energy emission likely in Klein-Nishina regime, making inverse-Compton less viable.

Detection of 70 GeV photons supports Bohm diffusive shock acceleration.

Abstract

Fermi observations of high-energy gamma-ray emission from GRB 080916C shows that its spectrum is consistent with the Band function from MeV to tens of GeV. Assuming one single emission mechanism dominates in the whole energy range, we show that this spectrum is consistent with synchrotron origin by shock-accelerated electrons. The simple electron inverse-Compton model and the hadronic model are found to be less viable. In the synchrotron scenario, the synchrotron self-Compton scattering is likely to be in the Klein-Nishina regime and therefore the resulting high-energy emission is subdominant, even though the magnetic field energy density is lower than that in relativistic electrons. The Klein-Nishina inverse-Compton cooling may also affect the low-energy electron number distribution and hence results in a low-energy synchrotron photon spectrum $n(\nu)\propto\nu^{-1}$ below the peak energy. Under the framework of the electron synchrotron interpretation, we constrain the shock microphysical parameters and derive a lower limit of the upstream magnetic fields. The detection of synchrotron emission extending to about 70 GeV in the source frame in GRB 080916C favors the Bohm diffusive shock acceleration if the bulk Lorentz factor of the relativistic outflow is not significantly greater than thousands.