Diffuse gamma-ray emission in the vicinity of open cluster Berkeley 87

Abstract Young massive stellar clusters have been recognized as a significant population of cosmic ray accelerators and gamma-ray sources. We report the detection of extended gamma-ray emission toward the young massive star cluster Berkeley 87 using 18 years of Fermi-LAT data. The emission has an angular extension of 0.36 degree and a photon index of 2.68. To investigate the ambient gas target, we combine CO, EBHIS and Planck data. The total gas mass within the region is ~21.61 × 102M⊙, with an average proton density of $\sim 368\ \rm cm^{-3}$. The hadronic scenario is favored given the dense gas and the cluster’s strong stellar winds. The kinetic energy injection rate from stellar winds in Berkeley 87 exceeds $1.0 \times 10^{36}\ \rm erg \ s^{-1}$. Considering a CR acceleration efficiency 10−2.0 to 10−2.5, the cluster can supply sufficient energy to power the observed gamma-ray emission. A nearby pulsar, PSR J2021+3651, associated with the Dragonfly pulsar wind nebula and the 4HWC source 4HWC J2021+3650, lies in the same field of view. We also discuss the maximum accelerated proton energy and electron energy density of this pulsar.