Unidentified Gamma-Ray Sources

When any region of the electromagnetic spectrum is opened to new investigation, or detectors suddenly achieve a marked improvement in sensitivity, an exciting era of discovery inevitably ensues. This is particularly true in the gamma-ray regime, where the launch of each new satellite mission has resulted in the discovery of previously 

The vast majority of the unidentified gamma-ray sources ( 120) appear to belong to a Galactic population. Many of these are likely to be obscured objects whose exact properties are masked by the Galactic diffuse radiation, or cannot be accurately distinguished from those of other nearby sources in the crowded disk of the Milky Way. Gamma rays from the unidentified sources are presumed 

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Annihilation fluxes would be enhanced, to a level where such sources could possibly account for a subset of the unidentified EGRET gamma ray sources. 

A compelling motivation for the search of gamma ray sources in the energy range between 30 GeV and 300 GeV is the apparent discrepancy between the well populated sky-map of sources below 10 GeV observed by EGRET (more than half of them still unidentified) and the handful of sources discovered by... 

There is not yet, however, a definite detection of diffuse gamma-ray emission from galaxy clusters. While there is a preliminary evidence of gamma-ray emission from a dozen bright, radio-active clusters which host powerful radio galaxies and Blazars and are associated to unidentified EGRET sources (Co-lafrancesco 2002), many of the quiet, X-ray selected clusters only have upper limits for their emission at E > 100 MeV. 

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