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The long gamma-ray burst GRB 191221B has abundant observations in X-ray,optical and radio bands. In the literature, the observed optical light curve ofGRB 191221B displays a plateau around 0.1-day, which is rather peculiar ingamma-ray bursts. Here we performed detailed analysis of the observational datafrom Swift/UVOT, VLT and LCO, obtained the light curve of the multi-bandafterglow of GRB 191221B. By examining optical, ultraviolet, X-ray, and radiodata for this event, we demonstrate that an on-axis two-component jet model canexplain the observations. Our analysis suggests that the narrow component hasan initial Lorentz factor of 400 and a jet opening half-angle of $1.4^{\circ}$,while the wide component has an initial Lorentz factor of 25 and a jet openinghalf-angle of $2.8^{\circ}$. The narrow jet dominates the early decay, whereasthe wider jet causes the optical plateau and dominates late decay. According tothis model, the reason for the absence of the X-ray plateau is due to thesteeper spectral index of the wide component, resulting in a less significantflux contribution from the wide jet in the X-ray bands than in the opticalbands. Moreover, we have explained the inconsistency in the decay indices ofthe UVOT and Rc-band data around 2000 seconds using reverse shock emission.

A two-component jet model for the optical plateau in the afterglow of GRB 191221B