2014MNRAS.438..752G

We use high-quality, multiband observations of Swift GRB 120404A, from γ-ray to radio frequencies, together with the new hydrodynamics code of van Eerten et al. to test the standard synchrotron shock model. The evolution of the radio and optical afterglow, with its prominent optical rebrightening at trest ∼ 260-2600s, is remarkably well modelled by a decelerating jet viewed close to the jet edge, combined with some early re-energization of the shock. We thus constrain the geometry of the jet with half-opening and viewing angles of 23° and 21°, respectively, and suggest that wide jets viewed off-axis are more common in GRBs than previously thought. We also derive the fireball microphysics parameters εB = 2.4x10^–4 and εe = 9.3x10^–2 and a circumburst density of n = 240/cm3. The ability to self-consistently model the microphysics parameters and jet geometry in this way offers an alternative to trying to identify elusive canonical jet breaks at late times. The mismatch between the observed and model-predicted X-ray fluxes is explained by the local rather than the global cooling approximation in the synchrotron radiation model, constraining the microphysics of particle acceleration taking place in a relativistic shock and, in turn, emphasizing the need for a more realistic treatment of cooling in future developments of theoretical models. Finally, our interpretation of the optical peak as due to the passage of the forward shock synchrotron frequency highlights the importance of high-quality multiband data to prevent some optical peaks from being erroneously attributed to the onset of fireball deceleration.