Gamma-ray burst spectral evolution through cross-correlations of discriminator light curves

Gamma-ray burst spectra usually show hard-to-soft evolution within intensity spikes and from spike to spike. The techniques used to study spectral evolution often lack sufficient temporal resolution to determine the nature of this evolution. By comparing the auto-and cross-correlations between the time histories of the BATSE large area detector discriminator rates, I have characterized the spectral evolution of a sample of 209 bursts. I find that hard-to-soft evolution is ubiquitous and that only similar to 10% of the bursts show clear soft-to-hard evolution.