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N.M. Lloyd, V. Petrosian (Stanford University)
We present the wide variety of spectral evolution present in a sample of about 80 GRBs, and attempt to categorize these bursts (both by eye, and using new methods involving the cross-correlation function between the time histories of the spectral parameters) according to their evolutionary behavior. We discuss in particular the correlations observed between the various spectral parameters; we show that a majority of the correlations are primarily due to instrumental effects, but that the physical processes involved also play a significant role. We present a physical explanation for the temporal evolution of the spectral parameters, in terms of synchrotron emission from internal shocks. In particular we show that allowing for a ``smooth cutoff'' to the electron energy distribution, an anisotropic distribution for the electron pitch angles, and - in a few cases - synchrotron self-absorption, synchrotron emission can explain the time resolved behavior of the spectral parameters throughout a burst, if we regard each pulse a separate/independent emission episode. Finally, we briefly discuss how the presence of other emission mechanisms may affect our model of emission from GRBs.
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