Low-latitude gamma-ray sources and the hypothesis of a black hole population

We present a study of 40 unidentified 3EG gamma-ray sources which were found to be not positionally coincident with any known class of galactic objects in a previous paper (Romero, Benaglia & Torres, 1999). We have performed a variability analysis which reveals that most of these 40 sources are variable. These sources have, in addition, a steep mean value of the gamma-ray spectral index, $<\Gamma> = 2.41 \pm 0.2$, which, combined with the high level of variability rules out a pulsar origin. The positional coincidences with uncatalogued candidates to supernova remnants were also studied. Only 7 sources in the sample are spatially coincident with these candidates, a result that is shown to be consistent with the expected level of pure chance association. A complementary search for weak radio counterparts was also conducted and the results are presented as an extensive table containing all significant point-like radio sources within the 40 EGRET fields. We argue that in order to produce the high variability, steep gamma-ray spectra, and absence of strong radio counterparts observed in the gamma-ray sources of our sample a new class of objects should be postulated, and that the only known candidate capable to generate such features are isolated magnetized black holes.