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

Abstract

We investigated the MeV properties of 173 unidentified or only 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 generally 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), 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.

arXiv comment9 pages including 4 figures; A&A accepted

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• XMM-Newton observations reveal the X-ray counterpart of the very-high-energy gamma-ray source HESSJ1640-465
• A microquasar model applied to unidentified gamma-ray sources
• A luminosity constraint on the origin of unidentified high energy sources
• Comptonization of cosmic microwave background by cold ultra-relativistic electron-positron pulsar wind and origin of ~100 GeV lines
• Microquasar models for 3EG J1828+0142 and 3EG J1735-1500