  Using a theoretical model describing pulse shapes, we have clarified the
relations between the observed pulses and their corresponding timescales, such
as the angular spreading time, the dynamic time as well as the cooling time. We
find that the angular spreading timescale caused by curvature effect of
fireball surface only contributes to the falling part of the observed pulses,
while the dynamic one in the co-moving frame of the shell merely contributes to
the rising portion of pulses provided the radiative time is negligible. In
addition, the pulses resulted from the pure radiative cooling time of
relativistic electrons exhibit properties of fast rise and slow decay (a
quasi-FRED) profile together with smooth peaks. Besides, we interpret the
phenomena of wider pules tending to be more asymmetric to be a consequence of
the difference in emission regions. Meanwhile, we find the intrinsic emission
time is decided by the ratios of lorentz factors and radii of the shells
between short and long bursts. Based on the analysis of asymmetry, our results
suggest that the long GRB pulses may occur in the regions with larger radius,
while the short bursts could locate at the smaller distance from central
engine.
