2005A&A...437L..23T

We report early BVR_cI_c imaging of the INTEGRAL GRB 041218. The observation started 129 s after the burst and a sequence of exposures in V, R_c, I_c, and B bands were repeated. The optical afterglow is detected in R_c and I_ c_ 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., 1998ApJ...497L..17S) and we crudely constrain the physical condition of the relativistic shock.