An INTEGRAL/SPI view of reticulum II: Particle dark matter and primordial black holes limits in the MeV range
Author
Siegert, Thomas
Boehm, Celine
Calore, Francesca
Diehl, Roland
Krause, Martin G. H.
Serpico, Pasquale D.
Vincent, Aaron C.
Attention
2299/25296
Abstract
Reticulum II (Ret II) is a satellite galaxy of the Milky Way 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 $\gamma$-ray band 25--8000 keV, and we derive a flux limit of $\lesssim 10^{-8}\,\mathrm{erg\,cm^{-2}\,s^{-1}}$. The previously reported 511 keV line is not seen, and we find a flux limit of $\lesssim 1.7 \times 10^{-4}\,\mathrm{ph\,cm^{-2}\,s^{-1}}$. We construct spectral models for primordial black hole (PBH) evaporation and annihilation/decay of particle DM, and subsequent annihilation of positrons produced in these processes. We exclude that the totality of DM in Ret II is made of a monochromatic distribution of PBHs of masses $\lesssim 8 \times 10^{15}\,\mathrm{g}$. Our limits on the velocity-averaged DM annihilation cross section into $e^+e^-$ are $\langle \sigma v \rangle \lesssim 5 \times 10^{-28} \left(m_{\rm DM} / \mathrm{MeV} \right)^{2.5}\,\mathrm{cm^3\,s^{-1}}$. 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 Milky Way, and encourage follow-up observations of Ret II and other dwarf galaxies.