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. © 2009. The American Astronomical Society. All rights reserved. Printed in the U.S.A.