Clues from the Prompt Emission of GRB 080319B

Abstract

The extremely bright optical flash that accompanied GRB 080319B suggested, at first glance, that the prompt γ-rays in this burst were produced by synchrotron self-Compton (SSC). We analyze here the observed optical and γ spectra. We find that the very strong optical emission imposes, due to self-absorption, very strong constraints on the emission processes and puts 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 γ-rays. We also show that the optical emission and the γ-rays could not have been produced by synchrotron emission from two populations of electrons within the same emitting region. Thus, we must conclude that the optical emission and the γ-rays were produced in different physical regions. A possible interpretation of the observations is that the γ-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 γ-ray signals.

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