Clues from the prompt emission of GRB 080319B

TL;DR

The paper analyzes the optical and gamma-ray spectra of GRB 080319B, concluding that the optical and gamma-ray emissions originated from different regions, with gamma-rays from internal shocks and optical from external shocks, challenging initial SSC assumptions.

Contribution

It provides constraints on emission processes in GRB 080319B, showing that optical and gamma-ray emissions are produced in separate regions, and rules out SSC as the source of prompt gamma-rays.

Findings

Optical emission is constrained by self absorption, indicating a large emission radius.

SSC cannot account for the prompt gamma-ray production.

Optical and gamma-ray emissions originate from different physical regions.

Abstract

The extremely bright optical flash that accompanied GRB 080319B suggested, at first glance, that the prompt $\gamma$-rays in this burst were produced by Synchrotron self Compton (SSC). We analyze here the observed optical and $\gamma$ spectrum. We find that the very strong optical emission poses, due to self absorption, very strong constraints on the emission processes and put the origin of the optical emission at a very large radius, almost inconsistent with internal shock. Alternatively it requires a very large random Lorentz factor for the electrons. We find that SSC could not have produced the prompt $\gamma$-rays. We also show that the optical emission and the $\gamma$ rays could not have been produced by synchrotron emission from two populations of electron within the same emitting region. Thus we must conclude that the optical and the $\gamma$-rays were produced in different physical regions. A possible interpretation of the observations is that the $\gamma$-rays arose from internal shocks but the optical flash resulted from external shock emission. This would have been consistent with the few seconds delay observed between the optical and $\gamma$-rays signals.