Point Sources of GeV Gamma Rays

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Abstract
A catalog of γ-ray sources based on photons with energies greater than 1 GeV has been developed from observations taken by the EGRET instrument of the Compton Gamma Ray Observatory. The data are taken from the 4.5 yr of observation available in the public data archives. We emphasize sources that are detected using the entire database, without regard to any possible transient or variable behavior. Ten of the 57 sources reported here have not previously been reported in the catalogs developed using photons above 100 MeV in energy. Twenty-seven sources have identifications with objects seen at other wavelengths: the Large Magellanic Cloud, five pulsars, and 21 blazars. The remaining 30 sources are classified as unidentified; however, seven may be associated with Galactic supernova remnants and one source may be a Galactic X-ray binary (LSI 61 303). The 30 unidentified sources are distributed nearly uniformly along the Galactic plane and are symmetric about it. Only one of the unidentified sources has a Galactic latitude in excess of 30°, whereas, if the sources were distributed uniformly, ~12 would be expected on the basis of the combined EGRET exposure. A scatter plot of the flux from the unidentified sources versus Galactic latitude reveals two rather distinct categories of source: "bright" sources with fluxes greater than or equal to 4.0 × 10-8 photons cm-2 s-1 and "dim" sources with fluxes of less than 4.0 × 10-8 photons cm-2 s-1. The absence of high-latitude bright sources is striking. The bright unidentified sources have an average Galactic latitude of 27, which is consistent with a Population I distribution at distances of 1-5 kpc. The dim unidentified sources have a broader latitude distribution with an average |b| = 138, indicating that if they are at the same average distance from the Galactic plane as the bright sources, they are paradoxically approximately 5 times closer than the bright objects on average and therefore roughly 2 orders of magnitude less luminous.