Application of the V/V_max_ test to PHEBUS gamma-ray bursts

The spatial homogeneity of Gamma-Ray Burst (GRB) sources observed by the PHEBUS experiment on board the GRANAT satellite is analysed using the V/V_max_ test. Since the launch of the satellite (December 1989), more than one hundred GRBs have been recorded in the first three years of operation. We obtain an average value of <V/V_max_>=0.37 +/- 0.028 which is incompatible with the hypothesis of a uniform distribution of bursters at a >4.5σ confidence level. This result is in good agreement with observations made by other instruments and implies that, in spite of its energy threshold above 120 keV, PHEBUS samples essentially the same burst population. However, the PHEBUS <V/V_max_> is relatively low compared to those obtained by SIGNE, KONUS or BATSE experiments which have a larger burst detection rate and therefore observe an a priori larger sampling volume. We investigate the possible origins of this difference and show that high-energy experiments generally display lower < V/V_max_> values than low-energy instruments, at comparable burst detection rates. The implications of this result are discussed.