Pulse width evolution in gamma-ray bursts: Evidence for internal shocks

Many cosmological models of gamma-ray bursts (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 find that the width of the large-amplitude pulses in GRB time histories remains remarkably constant throughout the classic GRB phase. This is also true for small-amplitude pulses. However, small and large pulses do not have the same pulse width within a single time history. We find a quantitative relationship between pulse amplitude and pulse width: the smaller amplitude peaks tend to be wider, with the pulse width following a power law with an index of about -2.8. Internal shocks simulated by randomly selecting the Lorentz factor and energy per shell are consistent with a power-law relationship. This is strong quantitative evidence that GRBs are indeed caused by internal shocks. The dependency of the width-versus-intensity relationship on the maximum Lorentz factor provides a way to estimate that elusive parameter. Our observed power-law index indicates that Gamma(max) is less than or similar to 10(3). We also interpret the narrowness of the pulse width distribution as indicating that the emission that occurs when one shell overtakes another is produced over a small range of distances from the central site.