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Aims. We report on a detailed study of the optical afterglow of GRB 061121with our original time-series photometric data. In conjunction with X-rayobservations, we discuss the origin of its optical and X-ray afterglows.Methods. We observed the optical afterglow of Swift burst GRB 061121 with theKanata 1.5-m telescope at Higashi-Hiroshima Observatory. Our observation coversa period just after an X-ray plateau phase. We also performed deep imaging withthe Subaru telescope in 2010 in order to estimate the contamination of the hostgalaxy. Results. In the light curve, we find that the optical afterglow alsoexhibited a break as in the X-ray afterglow. However, our observation suggestsa possible hump structure or a flattening period before the optical break inthe light curve. There is no sign of such a hump in the X-ray light curve.Conclusions. This implies that the emitting region of optical was distinct fromthat of X-rays. The hump in the optical light curve was possibly caused by thepassage of the typical frequency of synchrotron emission from another forwardshock distinct from the early afterglow. The observed decay and spectralindices are inconsistent with the standard synchrotron-shock model. Hence, theobservation requires a change in microphysical parameters in the shock regionor a prior activity of the central engine. Alternatively, the emission duringthe shallow decay phase may be a composition of two forward shock emissions, asindicated by the hump structure in the light curve.

Optical Behavior of GRB 061121 around its X-Ray Shallow Decay Phase