Diffuse gamma-ray emission from the vicinity of young massive star cluster RSGC 1

Abstract: We report the Fermi Large Area Telescope (Fermi-LAT) detection of the gamma-ray emission towards the young massive star cluster RSGC 1. Using the latest source catalog and diffuse background models, we found that the diffuse gamma-ray emission in this region can be resolved into three different components. The GeV gamma-ray emission from the region HESS J1837-069 has a photon index of 1.83 $\pm$ 0.08. Combining with the HESS and MAGIC data, we argue that the gamma-ray emission in this region likely originate from a pulsar wind nebula (PWN). The gamma-ray emission from the northwest part (region A) can be modelled by an ellipse with the semimajor and semiminor axis of 0.5$^{\circ}$ and 0.25$^{\circ}$, respectively. The GeV emission has a hard spectrum with a photon index of 2.05 $\pm$ 0.02 and is partially coincide with the TeV source MAGIC J1835-069. The possible origin of the gamma-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 gamma-ray emission from the southeast region (region B) can be modeled as an ellipse with the semimajor and semiminor axis of 0.9$^{\circ}$ and 0.5$^{\circ}$, respectively, and also reveals a hard gamma-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 cosmic-ray (CR) proton energy is estimated to be as high as $\sim$ 1x10$^{50}$ erg.