The hardness-duration diagram of gamma-ray bursts

The hardness-duration diagram for gamma-ray bursts still contains undiscovered important information about the intrinsic properties of these phenomena, We analyze these diagrams for the PHEBUS and BATSE experiments. First, we show that the BATSE diagram is very similar to that for PHEBUS when we restrict the BATSE data set to events observable by PHEBUS. In this case, both diagrams present a high degree of clustering into two subclasses. This shows that the brightness of the events is a more important factor in determining the aspect of this diagram than the hardness ratio energy ranges. Second, for the subclass of long bright bursts, both experiments show evidence for a positive correlation between hardness and duration. This is a significant new result, as it represents an intrinsic property of long events. The commonly held perception of an anticorrelation (between hardness and duration) for all bursts must be replaced by a more complicated model that treats the two subclasses separately. No statistically significant correlation is found within the short-event population. The existence of such all intrinsic positive correlation for the long bursts would complicate the search for cosmological effects in the gamma-ray burst data.