1520024

Koposov et al. (2015) used data from the Dark Energy Survey to search for new dwarf satellite galaxies (DSGs) of the Milky Way (MW). They found nine candidates in the southern sky which may be related to the magellanic systems. Among their sample Reticulum II (Ret II) is the closest and was detected with a significance of more than 48 sigma. Looking for a possible low energy gamma-ray emission from these new DSGs Siegert et al. (2016) found a 3.1 sigma signal at photon energies around 511 keV from the direction of Ret II with a flux of 1.7E-4 ph/cm2/s. The origins of such a 511 keV signal from electron-positron annihilations are versatile and probably not attributable to one particular source. The connection between this excess and the galactic centre signal remains unexplored. Massive stars and supernovae have been considered as a possible source of 511 keV photons in the MW since they can produce beta+-unstable isotopes. However they are unlikely to be responsible for the signal in Ret II since we do not expect any in DSGs. Dark matter (DM) has also been proposed as a possible source of low-energy positron emitters in the MW and could be a source of positrons in Ret II as it is dominated by DM. However the cross section needed to explain the 511 keV signal in Ret II would make the MW brighter at these energies. Another source of low-energy positrons could be microquasars. These may produce electron-positron pair-plasma predominantly in outbursts. In fact one single microquasar in Ret II might be enough to explain the whole emission. In this proposal we ask for 1.5 Ms of observation time towards the direction of Ret II (l/b) = (266.3/-49.7) which would add to the currently available 250 ks. This observation would consolidate or falsify the discovery of a 511 keV line signal in Ret II and understand its origin if it is indeed present. In addition observing these DSGs with SPI may help to solve the 40 years [truncated! Please see actual data for full text]