XMM-Newton and ESO observations of the two Unidentified γ-ray Sources 3EG J0616 3310 and 3EG J1249 8330 ⋆

The limited angular resolution of -ray telescopes prevents a direct identification of the majo rity of sources de- tected so far. This is particularly true for the low latitude , probably galactic, ones only 10 % of which has been identifie d. Most counterparts of the identified low-latitude -ray sources are Isolated Neutron Stars (INS), both radio-loud and radio- quiet (Geminga-like) objects, which are characterised by an extremely high value of the X-ray-to-optical flux ratio fX/fopt. Therefore, the systematic X-ray and optical coverage of low-latitude unidentified -ray sources aiming at high fX/fopt sources seems one of the most promising ways to spot INS candidate counterparts. Since low latitude sources are heavily affected by the interstellar absorption at both X-ray and optical wavel engths, we have focussed on two middle-latitude, probably galactic, GRO/EGRETsources: 3EG J0616 3310 and 3EG J1249 8330. These two sources, which could belong to a local galactic population, have been selected owing to their relatively good positional accuracy, spectral shape and lack of candidate extra- galactic radio counterparts. Here we report on X-ray observations of the two -ray error boxes performed with XMM-Newton and on their optical follow-up carried on with the Wide Field Imager at the ESO/MPG 2.2m telescope. Less than half of the � 300 sources detected by the X-ray coverage have no optical counterparts. Among those, we have selected few interesting sources with fX/fopt � 100, which we consider promising INS candidates.