GROSS SPECTRAL DIFFERENCES BETWEEN BRIGHT AND DIM GAMMA-RAY BURSTS

We find that dim gamma-ray bursts (GRBs) are softer than bright GRBs, as indicated on average by data from the Burst and Transient Source Ex periment (BATSE) on board the Compton Gamma Ray Observatory. We show t hat this correlation is statistically significant with respect to vari ations due to random differences between GRBs. This effect is discerni ble using a variety of methods and data sets, including public domain data. We analyze several types of systematic errors and selection effe cts in the BATSE data and conclude that the observed effect is not dom inated by any of them. We therefore assert that this dim/soft effect i s a real property of GRBs. It is possible that this correlation is a c onsequence of the time dilation detected by Norris et al. (1994) and t hat the burst sources are located at cosmological distances.