A dark cloud associated with an unidentified EGRET source

CO J = 1-0 mapping observations of the dark nebula Lynds 227, which is possibly associated with the unidentified EGRET source 2EG J1811-2339, are presented. We detected a large amount of molecular gas along Lynds 227 with a total mass of (1-2) × 10⁴ M_⊙ surrounding an X-ray synchrotron nebula, which was detected by the ASCA within the error circle of 2EG J1811-2339. Molecular gas along Lynds 227 shows spatial anticorrelation with the X-ray synchrotron nebula, suggesting that the synchrotron nebula is interacting with the Lynds 227 cloud. We propose a γ-ray emission mechanism for 2EG J1811-2339: high-energy electrons are injected from a rotation-powered pulsar and further accelerated in shock waves generated by the interaction with ambient matter. The high-energy electrons move into the molecular cloud at the Lynds 227 and collide with dense interstellar matter to yield high-energy γ-ray photons, mainly through relativistic bremsstrahlung.