Point Sources of GeV Gamma - Rays

A catalog of gamma-ray sources based on photons with energies greater than 1 GeV has been developed from observations taken by the EGRET instrument o f the Compton Gamma Ray Observatory. The data are taken from the 4.5 years of observation available in the public data archive. We emphasize sources which are detected using the entire database without regard to any possible transient o r v ariable behavior. Ten of the 57 sources reported here have not previously been reported in the catalogs developed using photons above 100 MeV in energy Thompson et al. 1995, 1996. Twenty-seven sources have identiications with objects seen at other wavelengths: the Large Magellanic Cloud, ve pulsars, and 21 blazars. The remaining 30 sources are classiied as unidentiied, however 7 may be associated with galactic supernova remnants Esposito et al. 1996; Sturner, Dermer & Mattox 1996 and one source may be a galactic X-ray binary LSI 61 303. The 30 unidentiied sources are distributed nearly uniformly along the galactic plane and symmetric about it. Only one of the unidentiied 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 ux from the unidentiied sources versus galactic latitude reveals two rather distinct categories of source: bright" sources with uxes 4:0 10 ,8 photonsscm 2 s and dim" sources with uxes 4:0 10 ,8 photonsscm 2 s. The absence of high latitude, bright sources is striking. The bright, unidentiied sources have a n a v erage galactic latitude of 2.7 consistent with a population I distribution at distances of 115 kpc. The dim, unidentiied sources have a broader latitude distribution with an average jbj = 1 3 : 8 indicating that, if they are at the same average distance from the galactic plane as the bright sources, they are paradoxically, o n a v erage, approximately 5 times closer than the bright objects and therefore roughly two orders of magnitude less luminous.