Unidentified Sources
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This summary view of the high-energy gamma-ray sky, in Galactic coordinates, shows many of the important features of the EGRET sky survey. In the background is the large-scale diffuse gamma radiation - a bright band along the Milky Way, produced by cosmic-ray collisions with gas and photon fields in our Galaxy, and an isotropic, presumably extragalactic component at high Galactic latitudes that may be the sum of unresolved distant sources. The 271 sources of the third EGRET catalog are shown as yellow symbols - identified sources are blazar Active Galactic Nuclei, pulsars, a solar flare, and the Large Magellanic Cloud. Over 60% of the third catalog sources remain unidentified. Some of these are clustered along the Galactic plane or around the Galactic Center, while others are scattered across the sky, suggesting at least two populations. Although searches continue at other wavelengths for counterparts to these mysterious sources, it seems likely that they will remain a legacy to the proposed Gamma-ray Large Area Space Telescope (GLAST).
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