Gamma-ray Burst 080319B: Evidence for Relativistic Turbulence, Not Internal Shocks

Abstract

We show that the excellent optical and gamma-ray data available for GRB 080319B rule out the internal shock model for the prompt emission. The data instead point to a model in which the observed radiation was produced close to the deceleration radius ($\sim10^{17}$ cm) by a turbulent source with random Lorentz factors $\sim10$ in the comoving frame. The optical radiation was produced by synchrotron emission from relativistic electrons, and the gamma-rays by inverse Compton scattering of the synchrotron photons. The gamma-ray emission originated both in eddies and in an inter-eddy medium, whereas the optical radiation was mostly from the latter. Therefore, the gamma-ray emission was highly variable whereas the optical was much less variable. The model explains all the observed features in the prompt optical and gamma-ray data of GRB 080319B. We are unable to determine with confidence whether the energy of the explosion was carried outward primarily by particles (kinetic energy) or magnetic fields. Consequently, we cannot tell whether the turbulent medium was located in the reverse shock (we can rule out the forward shock) or in a Poynting-dominated jet.