Temporal evolution of the pulse width in GRBs

Many cosmological models of GRBs envision the energy source to be a catacly smic stellar event leading to a relativistically expanding fireball. Partic les are thought to be accelerated at shocks and produce nonthermal radiatio n. The highly variable temporal structure observed in most GRBs has signifi cantly constrained models. By using different methods of statistical analys is in the time domain we show that the width of the pulses in GRBs time his tories 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 f rom 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 emitti ng shell that are substancially smaller than Gamma(-1). The lack of tempora l evolution of the pulse width should be explained by any fireball shock sc enario.