Search for the Possible Emission Regions for Different Categories of Gamma-ray Bursts Received Day Month Year Revised Day Month Year Communicated by Managing Editor

Using a theoretical model describing pulse shapes, we have clarified the relations between the observed pulses and their corresponding timescales, such as angular spreading time, dynamic time as well as 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 them provided the radiative time is negligible. In addition, the observed pulses resulted from the pure radiative cooling time of relativistic electrons exhibit the property 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 times of short bursts in the co-moving frame are larger than for long ones. 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.