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

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 of GRB 191221B displays a plateau around 0.1 d, which is rather peculiar in gamma-ray bursts. Here, we performed detailed analysis of the observational data from Swift/UVOT, Very Large Telescope, and Las Cumbres Observatory, obtained the light curve of the multiband afterglow of GRB 191221B. By examining optical, ultraviolet, X-ray, and radio data for this event, we demonstrate that an on-axis two-component jet model can explain the observations. Our analysis suggests that the narrow component has an initial Lorentz factor of 400 and a jet opening half-angle of 1.4 degrees, while the wide component has an initial Lorentz factor of 25 and a jet opening half-angle of 2.8 degrees. The narrow jet dominates the early decay, whereas the wider jet causes the optical plateau and dominates late decay. According to this model, the reason for the absence of the X-ray plateau is due to the steeper spectral index of the wide component, resulting in a less significant flux contribution from the wide jet in the X-ray bands than in the optical bands. Moreover, we have explained the inconsistency in the decay indices of the UVOT- and Rc-band data around 2000 s using reverse shock emission.