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3.2. The Angular Distribution

Even before these redshift measurements there was a strong evidence that GRBs originate from cosmological distances. The observed angular distribution is incompatible with a galactic disk distribution unless the sources are at distances less than 100 pc. However, in this case we would expect that <V / Vmax> = 0.5 corresponding to a homogeneous distribution [177] while the observations yield <V / Vmax> = 0.33.

A homogeneous angular distribution could be produced if the GRB originate from the distant parts of the galactic halo. Since the solar system is located at d = 8.5 kpc from the galactic center such a population will necessarily have a galactic dipole of order d / R, where R is a typical distance to a GRB [179]. The lack of an observed dipole strongly constrain this model. Such a distribution of sources is incompatible with the distribution of dark matter in the halo. The typical distance to the GRBs must of the order of 100 kpc to comply with this constraint. For example, if one considers an effective distribution that is confined to a shell of a fixed radius then such a shell would have to be at a distance of 100kpc in order to be compatible with current limits on the dipole [180].