Astron. Astrophys. 364, L93-L96 (2000)

1. Introduction

Above 100 MeV, EGRET has discovered 70 persistent, unidentified, sources at medium latitudes that clearly differ from the 40 sources seen close to the Galactic plane (Hartman et al. 1999): they are significantly softer, fainter, and have a steeper logN-logS function than at low latitudes (Gehrels et al. 2000). They have been tentatively associated with the local interstellar medium and the Gould Belt (Grenier 1997; Gehrels et al. 2000; Grenier 2000), but their nature remains a mystery. The likely candidates present in the Galactic disc, such as pulsars, supernova (SN) remnants, and OB associations, cannot account for so many sources off the plane. Interestingly, the starburst Gould Belt disc is tilted at to the Milky Way and close enough for weak sources to be detected. Proposing an origin of the sources in the Belt however requires one a) to confront their spatial distribution with that of the Belt and other likely Galactic structures given strong observational biases; b) to evaluate the number of likely -ray emitters, i.e. compact stars, formed in the Belt in the recent past. Both aspects are analyzed below before discussing the possible nature of the sources.

© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001
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