TP-28

An Analysis of Pulse Spectral Evolution: Observations Vs. Internal Shock Model Predictions

A. M. Soderberg & E. E. Fenimore (LANL)

Many GRB models postulate the existence of superrelativistic expanding shells originating from a central explosion site. Internal shocks in the flow convert bulk motion into radiation. The shells collide with one another releasing approximately 10⁵¹ ergs/s^-1. The rise of the resulting pulse shape is dependent on the time the shell emits whereas the decay of the pulse relies only on the curvature of the shell. We compare the spectral evolution of the decay of individual GRB pulses to that expected from curvature. In particular, we examine the relationship between photon flux (I) and the peak of the $\nu$F$\nu$ distribution (Epeak) as predicted by the internal shock model. Kinematics necessitate that Epeak has a power-law relationship to I which is defined as: Epeak(I)=I $^{(1/\alpha+1))}$where $\alpha$ is the index of the photon number spectrum. Preliminary analyses demonstrate that a sizable fraction of observed pulses have spectra which evolve faster than kinematics allow. The relationship evident between Epeak and I in the decay phase is robust. As a result, models must confront the fact that pulses do not seem to evolve as predicted by the internal shock model.