The Hardness-Intensity Correlation And The Spatial Distribution Of Grbs

We demonstrate the presence of a hardness-intensity correlation (HIC) in a sample of 77 bright GRBs detected by Phebus/GRANAT and the Ulysses GRB detector. Using simulations, we find that the peak spectral hardness and the peak intensity in Phebus are correlated at a 5 sigma confidence level. This significant correlation is associated with a [V/V-max]= 0.5 in Ulysses. These two results together imply several constraints on the spatial distribution of bursters. In a cosmological scenario, the apparent contradiction between the presence of a RIG, requiring high redshifts, and [V/V-max]= 0.5, characteristic of nearby bursters, can be resolved by assuming a density evolution of sources. In this case our calculations show that the typical bright sources observed by Phebus and Ulysses should be at z(max) approximate to 2 - 3 to explain the strength of the hardness-intensity correlation.