An External Shock Origin of GRB $\textit{141028A}$

TL;DR

This paper demonstrates that the prompt emission of GRB 141028A can be explained by an external shock model involving synchrotron emission, challenging the traditional separation between prompt and afterglow phases.

Contribution

It introduces an external shock model fitting the prompt emission of GRB 141028A, showing that both prompt and afterglow emissions can originate from the same external shock process.

Findings

The gamma-ray spectrum fits a synchrotron+blackbody model.

The synchrotron peak evolution matches the analytic external shock model.

The external shock model can reproduce observed flux densities within an order of magnitude.

Abstract

The prompt emission of the long, smooth, and single-pulsed gamma-ray burst, GRB $\textit{141028A}$, is analyzed under the guise of an external shock model. First, we fit the $\gamma$-ray spectrum with a two-component photon model, namely synchrotron+blackbody, and then fit the recovered evolution of the synchrotron $\nu F_{\nu}$ peak to an analytic model derived considering the emission of a relativistic blast-wave expanding into an external medium. The prediction of the model for the $\nu F_{\nu}$ peak evolution matches well with the observations. We observe the blast-wave transitioning into the deceleration phase. Further we assume the expansion of the blast-wave to be nearly adiabatic, motivated by the low magnetic field deduced from the observations. This allows us to recover within an order of magnitude the flux density at the $\nu F_{\nu}$ peak, which is remarkable considering the simplicity of the analytic model. Across all wavelengths, synchrotron emission from a single forward shock provides a sufficient solution for the observations. Under this scenario we argue that the distinction between $\textit{prompt}$ and $\textit{ afterglow}$ emission is superfluous as both early and late time emission emanate from the same source. While the external shock model is clearly not a universal solution, this analysis opens the possibility that at least some fraction of GRBs can be explained with an external shock origin of their prompt phase.