Temporal evolution (black lines) of the Lorentz factor for the narrow (left panel) and wide (right panel) jet components. The red-dotted lines are plotted for comparison and correspond to the standard evolution Γ ∝ t ^−3/8 for an adiabatic fireball expanding in a uniform medium (e.g., Sari et al. 1998). The initial values Γ ₀ are set to be 10 ⁴ and 220 for the narrow and wide jet components, respectively. After t _j , which is set to be ~1000 s for the narrow jet (corresponding to θ _j ~ 0 1) and ~2 × 10 ⁴ s for the wide jet (corresponding to θ _j ~ 0 4), the Lorentz factor is evolved following a temporal scaling of Γ ∝ t ^−1/2 (e.g., Peng et al. 2005).