Letter to the Editor

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

Department of Earth and Space Science, Graduate School of Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan e-mail: torii@ess.sci.osaka-u.ac.jp

Received: 16 March 2005
Accepted: 3 May 2005

Abstract

We report early 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  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, ApJ, 497, L17) and we crudely constrain the physical condition of the relativistic shock.