Spectral constraints on unidentified EGRET gamma-ray sources from COMPTEL MeV observations

We investigated the MeV properties of 173 unidentified or tentatively identified EGRET sources listed in the third EGRET catalogue by analyzing the simultaneously collected COMPTEL MeV data for each individual source. The sources can be divided into 4 groups. In this paper we focus on one of these, a group of 22 EGRET sources for which we can provide additional constraining information: their spectral extrapolations from the energy range above 100 MeV towards lower energies overshoot the fluxes or upper limits derived simultaneously at MeV energies. This means that for these sources a spectral turnover/break between 1 MeV and 100 MeV is required. At least two of these sources, but most likely the majority of this sample, have the maxima of their gamma-ray luminosities in this energy band. The sources have rather soft EGRET spectra (average photon index = 2.72^+0.08_-0.11), and seem to spatially cluster in the inner Galaxy. Variability analyses revealed 11 out of the 22 sources to be significantly variable. Object classes proposed as possible counterparts for the unidentified EGRET sources are discussed in the light of these additional constraints.