EVIDENCE FOR A GALACTIC GAMMA-RAY HALO

We present quantitative statistical evidence for a gamma-ray emission halo surrounding the Galaxy. Maps of the emission are derived. EGRET d ata were analyzed in a wavelet-based non-parametric hypothesis testing framework, using a model of expected diffuse (Galactic + isotropic) e mission as a null hypothesis. The results show a statistically signifi cant large scale halo surrounding the center of the Milky Way as seen from Earth. The halo flux at high latitudes is somewhat smaller than t he isotropic gamma-ray flux at the same energy, though of the same ord er (O(10(-7)-10(-6)) ph cm(-2) s(-1 )sr(-1) above 1 GeV). (C) 1998 Els evier Science B.V. All rights reserved.