2020005

There is an ongoing debate about the origin of the 511 keV electron-positron signal: Many unresolved point sources or truly diffuse emission in the interstellar medium. A smooth distribution of sub-threshold sources has been suggested in two alternative scenarios: Bisnovatyi-Kogan et al. (2017) focused on the cumulative effect of intermittently flaring stars producing a 511 keV line through the same processes as in the Sun. Coronal mass ejections carry accelerated particles that eventually produce positrons through decays which subsequently annihilate in the solar atmosphere. Bartels et al. (2018) proposed the idea of accreting compact objects in quiescence that steadily eject sub-relativistic pair-plasma which may also be related to the unresolved GeV Galactic centre excess.Both studies suggest that their models could be readily tested through the observation of globular clusters (GCs) at 511 keV. In both cases the predicted 511 keV flux from individual GCs is too low for SPI to be detectable however the whole Milky Way population of 157 GCs is estimated to add up to 10% of the diffuse flux. The relative 511 keV fluxes from GCs would then allow to distinguish between models providing a smoking-gun evidence for the true origin of Galactic positrons.Most GCs are located in the same regions where the diffuse 511 keV flux from bulge and disk is also strong so that individual point sources will not outshine the diffuse emission. For this reason we proposed and were granted long-term SPI observations in AO18 and AO19 to target a region that includes GCs which would show a high contrast against the diffuse emission. In AO20 we wish to continue these observations to reach our goal of quadrupling the exposure time in the chosen region within three years. Based on detailed modelling to predict the 511 keV emission in GCs we found two neighbouring and (still) underexposed spots in the sky we wish to observe in four [truncated! Please see actual data for full text]