2022MNRAS.511..914S

Reticulum II (Ret II) is a satellite galaxy of the Milky Way (MW) and presents a prime target to investigate the nature of dark matter (DM) because of its high mass-to-light ratio. We evaluate a dedicated INTEGRAL observation campaign data set to obtain γ-ray fluxes from Ret II and compare those with expectations from DM. Ret II is not detected in the γ-ray band 25-8000 keV, and we derive a flux limit of ≲10^–8 erg cm^–2 s^–1. The previously reported 511 keV line is not seen, and we find a flux limit of ≲1.7 ×10^–4 ph cm^–2 s^–1. We construct spectral models for primordial black hole (PBH) evaporation and annihilation/decay of particle DM, and subsequent annihilation of e⁺s produced in these processes. We exclude that the totality of DM in Ret II is made of a monochromatic distribution of PBHs of masses ≲8 ×10¹⁵ g. Our limits on the velocity-averaged DM annihilation cross section into e⁺e^– are <σ v >≲5 ×10^–28 (m_DM/ MeV)^2.5 cm³s^–1. We conclude that analysing isolated targets in the MeV γ-ray band can set strong bounds on DM properties without multi-year data sets of the entire MW, and encourage follow-up observations of Ret II and other dwarf galaxies.