An INTEGRAL/SPI view of reticulum II: particle dark matter and primordial black holes limits in the MeV range

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 gamma-ray fluxes from Ret II and compare those with expectations from DM. Ret II is not detected in the y-ray band 25-8000 keV, and we derive a flux limit of less than or similar to 10(-8) erg cm(-2) s(-1). The previously reported 511 keV line is not seen, and we find a flux limit of less than or similar to 1.7 x 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 less than or similar to 8 x 10(15) g. Our limits on the velocity-averaged DM annihilation cross section into e(+)e(-) are <sigma v > less than or similar to 5 x 10(-28) (m(DM)/MeV)(2.5) cm(3) s(-1). We conclude that analysing isolated targets in the MeV gamma-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.