ASTRONOMY AND ASTROPHYSICS Temporal evolution of the pulse width in GRBs

Many cosmological models of GRBs envision the energy source to be a cataclysmic stellar event leading to a relativistically expanding fireball. Particles are thought to be accelerated at shocks and produce nonthermal radiation. The highly variable temporal structure observed in most GRBs has significantly constrained models. By using different methods of statistical analysis in the time domain we show that the width of the pulses in GRBs time histories remain remarkably constant throughout the classic GRB phase. If the emission sites lie on a relativistically expanding shell, we determine both the amount of deceleration and the angular spread of the emitting region from the time dependency of the pulse width. We find no deceleration over at least 2/3 of the burst duration and angular spreads of the complete emitting shell that are substancially smaller than Γ. The lack of temporal evolution of the pulse width should be explained by any fireball shock scenario.