Early BVRcIc imaging and the discovery of the optical afterglow of GRB 041218

Abstract

We report early BVRcIc imaging of the INTEGRAL GRB 041218. The observation started 129 s after the burst and a sequence of exposures in V, Rc, Ic, and B bands were repeated. The optical afterglow is detected in Rc and Ic bands while it was not detected in B and V. We find that the early afterglow deviates from a single power law decay and identify two characteristic timescales (breaks). Such breaks are theoretically expected from synchrotron cooling in the relativistic fireball (Sari et al. 1998) and we crudely constrain the physical condition of the relativistic shock.