Large-Scale Anisotropy of EGRET Gamma Ray Sources

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In the course of its operation, the EGRET experiment detected high-energy gamma ray sources at energies above 100 MeV over the whole sky. In this communication, we search for large-scale anisotropy patterns among the catalogued EGRET sources using an expansion in spherical harmonics, accounting for EGRET's highly non-uniform exposure. We find significant excess in the quadrupole and octopole moments. This is consistent with the hypothesis that, in addition to the galactic plane, a second mid-latitude (5∘ < |b| < 30∘) population, perhaps associated with the Gould belt, contributes to the gamma ray flux above 100 MeV.