Possible extragalactic positron annihilation signal

Context. With 20 years of INTEGRAL/SPI observations, the most detailed map of the positron annihilation line at 511 keV was created. While the central bulge and extended disk are readily recognised in this map, several hotspots at high latitude regions may either be imaging artefacts or true signals. Aims. We discuss possible extragalactic positron annihilation signals from hotspots in this map. We also calculate a cosmological positron annihilation signal as a contribution to the cosmic gamma-ray background (CGB). Methods. For this investigation, we compared 511 keV emission hotspots away from the Galactic plane with a high velocity cloud column density map as well as with the catalogue of local volume galaxies (LVGs) up to 25 Mpc. Results. In particular, the Magellanic Stream in the southern and Complex C in the northern sky match the brightest hotspots. This might indicate that the positron production rate inside the Milky Way is higher than measured from the Galactic interstellar medium alone, potentially up to 10 44 s −1 . In addition, we can explain other hotspots by the cumulative effect of LVGs in the selected regions. The CGB contribution from positron annihilation might be sub-dominant on the percent level. However, depending on the true intrinsic annihilation spectrum, in particular, depending on the positron injection energy for in-flight annihilation and the star formation rate per galaxy, a much higher imprint beyond 10% is possible above several MeV. Conclusions. If these findings are confirmed, next-generation MeV telescopes will, for the first time, identify individual extragalactic 511 keV sources. In particular, several dwarf spheroidal galaxies with fluxes of up to (1 − 2)×10 −5 ph cm −2 s −1 , the galaxies M31 and M33, and some of their satellites, with potentially several 10 −6 ph cm −2 s −1 each, may be detected.