Nonthermal X-ray “prompt emission” at E_obs = 1 keV in the observer’s frame, based on the extrapolated Band function. The jet duration is t_obs = 10 s for on-axis (θ_v = 0°) observers. In the on-axis lightcurves, the power-law decay at t_obs > 10 s arises from the high-latitude emission. The blue (θ_v = 0°), orange (θ_v = 6°), and green (θ_v = 8°) lightcurves correspond to different viewing angles. Solid and dashed lines are for different jet Lorentz factors of Γ_j = 300 and 100, respectively.