1820020

The 511 keV electron-positron annihilation signal from the bulge region of the Milky Way puzzles both observers and theorists. While INTEGRAL/SPI made great progress in characterising the emission and its spectral appearance there is an ongoing debate about the actual origin of the signal: Many unresolved point sources or truly diffuse emission from annihilation in the interstellar medium. A smooth distribution of subthreshold 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 positrons from decays. Bartels et al. (2018) proposed the idea of accreting compact objects in quiescence that steadily eject subrelativistic pair-plasma into their surroundings. These may also be related to the unresolved GeV emission which is thought to originate from a population of millisecond pulsars in the Galactic bulge. 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 flux from individual GCs is too low for SPI to be detectable however the whole Milky Way population is estimated to add up to more than 10% of the diffuse flux of 1e-3 ph/cm2/s. The relative fluxes from GCs would then allow to distinguish between models. Most GCs are located in the same regions where the diffuse 511 keV flux is also strong so that individual point sources will not outshine the background. For this reason we propose long-term observations to target a region that includes expectedly-strong GCs which would show a high contrast against the diffuse emission. Based on detailed modelling of a flaring star scenario and correlations from GeV emission to predict the 511 keV emission in GCs we propose a multi-year program to observe seven GCs in two neighbouring and underexposed spots in the sky for 500 ks each in AO18 and [truncated! Please see actual data for full text]