Imaging the positron annihilation line with 20 years of INTEGRAL/SPI observations

¹ Julius-Maximilians-Universität Würzburg, Fakultät für Physik und Astronomie, Institut für Theoretische Physik und Astrophysik, Lehrstuhl für Astronomie, Emil-Fischer-Str. 31, 97074 Würzburg, Germany
² The Hakubi Center for Advanced Research, Kyoto University, Yoshida Ushinomiyacho, Sakyo-ku, Kyoto 606-8501, Japan
³ Department of Physics, Kyoto University, Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
⁴ RIKEN Nishina Center, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
⁵ Kavli Institute for the Physics and Mathematics of the Universe (WPI), UTIAS, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8583, Japan

^★ Corresponding author: hiroki.yoneda.phys@gmail.com

Received: 11 June 2025
Accepted: 27 August 2025

Abstract

Context. The electron–positron annihilation line at 511 keV provides a unique probe for studying the distribution and origin of positrons in our Galaxy. The SPI spectrometer on board the International Gamma-Ray Astrophysics Laboratory (INTEGRAL) has observed this gamma-ray line for two decades.

Aims. We analyzed 20 years of INTEGRAL/SPI observations to produce the most sensitive all-sky map of the 511 keV line emission to date with the aim of revealing new features and providing refined measurements of known sources.

Methods. We performed image deconvolution using the Richardson-Lucy algorithm and employed a bootstrap analysis to evaluate statistical uncertainties of fluxes from regions of interest. Systematic uncertainties in the parameter choices for the image reconstruction were also considered. We utilized GPU acceleration to enable this computationally intensive data analysis.

Results. The reconstructed image successfully recovers the basic morphological features reported in model-fitting studies: a bright central component, a broad bulge, and an elongated disk component along the Galactic plane. We also report hints of new spatial features in the reconstructed image, including an asymmetric structure in the broad bulge emission and a 511 keV emission potentially associated with massive stars from the Scorpius–Centaurus and other OB associations. While the significance of these new features is marginal (~2σ), they are spatially consistent with ²⁶Al emission from massive stars in that region, suggesting that this 511 keV emission originates from its β⁺ decay.

Conclusions. Our 20-year dataset provides the most detailed 511 keV emission map to date, reproducing the global structures suggested in the model-fitting approach while also revealing hints of new spatial features. These findings provide new insights into the origin of Galactic positrons and the propagation of low-energy positrons in the interstellar medium. Future MeV gamma-ray observations, such as with COSI, are expected to confirm the reported features and shed further light on the nature of positrons in our Galaxy.