Point sources of GeV gamma rays

Abstract

A catalog of c-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 identiÐcations with objects seen at other wavelengths: the Large Magellanic Cloud, Ðve pulsars, and 21 blazars. The remaining 30 sources are classiÐed as unidentiÐed; however, seven may be associated with Galactic supernova remnants and one source may be a Galactic X-ray binary (LSI 61 303). The 30 unidentiÐed sources are distributed nearly uniformly along the Galactic plane and are symmetric about it. Only one of the unidentiÐed sources has a Galactic latitude in excess of 30¡, whereas, if the sources were distributed uniformly, D12 would be expected on the basis of the combined EGRET exposure. A scatter plot of the Ñux from the unidentiÐed sources versus Galactic latitude reveals two rather distinct categories of source: ““ bright ÏÏ sources with Ñuxes greater than or equal to 4.0]10~8 photons cm~2 s~1 and ““ dim ÏÏ sources with Ñuxes of less than 4.0]10~8 photons cm~2 s~1. The absence of high-latitude bright sources is striking. The bright unidentiÐed sources have an average Galactic latitude of 2¡.7, which is consistent with a Population I distribution at distances of 1È5 kpc. The dim unidentiÐed sources have a broader latitude distribution with an average o b o\13¡.8, 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 luminou