The hardness-intensity correlation in bright gamma-ray bursts

The presence of a hardness-intensity correlation (HIC) is demonstrated in a sample of 77 bright bursts observed by the Phebus and Ulysses gamma-ray burst (GRB) experiments. Using simulations, we find a significant correlation (5 sigma) between the peak spectral hardness and the peak intensity in Phebus. Moreover, the HIC is compatible with a single regression line over the full range of intensities. This result, together with earlier ones, shows that the HIC is a fundamental property of GRBs that has important consequences for statistical studies of these events. First, the presence of a HIC implies that the intensity distribution (and [V/V-max]) depends on the energy range of the instrument. Second, we show that the HIC, combined with [V/V-max], constrains the spatial distribution of the bursters. Bright events in our sample exhibit a significant HIC and [V/V-max] = 0.5. The interpretation of these observations is different for cosmological and galactic models. In Euclidean space, two features are required to explain the observations: bursters must belong to a homogeneous, bounded population and must have an intrinsic hardness-luminosity correlation. In a cosmological scenario, the apparent contradiction between the HIC (which requires high redshifts) and [V/V-max] = 0.5 (characteristic of nearby bursters) can be resolved by assuming a density evolution of the sources.