2020MNRAS.494.3405S

We report the Fermi Large Area Telescope (Fermi-LAT) detection of the γ-ray emission towards the young massive star cluster RSGC 1. Using the latest source catalogue and diffuse background models, we found that the diffuse γ-ray emission in this region can be resolved into three different components. The GeV γ-ray emission from the region HESS J1837-069 has a photon index of 1.83 ± 0.08. Combining with the HESS and MAGIC data, we argue that the γ-ray emission in this region likely originates from a pulsar wind nebula. The γ-ray emission from the north-west part (region A) can be modelled by an ellipse with the semimajor and semiminor axes of 0.5° and 0.25°, respectively. The GeV emission has a hard spectrum with a photon index of about -2 and partially coincides with the TeV source MAGIC J1835-069. The possible origin of the γ-ray emission in this region is the interaction of the cosmic rays (CRs) accelerated by SNR G24.7+0.6 or/and the OB cluster G25.18+0.26 with the surrounding gas clouds. The GeV γ-ray emission from the south-east region (region B) can be modelled as an ellipse with the semimajor and semiminor axes of 0.9° and 0.5°, respectively, and also reveals a hard γ-ray spectrum. We argue that the most probable origin is the interaction of the accelerated protons in the young massive star cluster RSGC 1 with ambient gas clouds, and the total CR proton energy is estimated to be as high as ∼1×10⁵⁰ erg.