  The long gamma-ray burst GRB 161017A was detected by Fermi and Swift, and its
afterglow was observed by the MITSuME 50-cm optical telescope promptly about 50
s after the burst. Early optical observations revealed that the optical
lightcurve exhibits a plateau and rebrightening in the early afterglow phase
about 500 and 5000 s after the trigger, respectively. By investigating the
behavior of the spectral and temporal flux variation, it was found that the
plateau and re-brightening cannot be explained in the context of the simple
standard afterglow model. These observational features can be explained with
two independent refreshed shocks, which indicate the long-acting central
engine. We evaluated the physical parameters of the subsequent shells, and we
then determined the kinetic energy ratio of the two colliding shells to the
leading shell to be roughly 1 and 8, respectively. In addition, two prominent
X-ray flares about 200 s after the trigger may be signatures of delayed
ejections of the energetic jets responsible for the refreshed shocks. Such late
activity of the central engine and X-ray flares play a crucial role in
understanding the mechanisms for jet formation and photon emission.
