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

Abstract

The e${}^{+}$ e${}^{-}$ annihilation line at 511 keV provides a unique probe for studying the distribution and origin of positrons in our Galaxy. The SPI spectrometer on INTEGRAL has observed this gamma-ray line for two decades. We analyze 20 years of INTEGRAL/SPI observations to produce the most sensitive all-sky map of the 511 keV line emission to date, aiming to reveal new features and provide refined measurements of known sources. We perform image deconvolution using the RL algorithm and employ bootstrap analysis to evaluate statistical uncertainties of fluxes from regions of interest. Systematic uncertainties in parameter choices are also considered. We utilize GPU acceleration to enable this computationally intensive analysis. 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 511 keV emission potentially associated with massive stars from the Sco-Cen and other OB associations. While the significance of these new features is marginal ($\sim 2\sigma$), they are spatially consistent with ${}^{26}$Al emission from massive stars in that region, suggesting that this 511 keV emission originates from its ${\beta }^{+}$ decay. Our 20-year dataset provides the most detailed 511 keV emission map to date, reproducing global structures suggested in model-fitting approach while revealing hints of new spatial features. These findings provide insights into the origin of Galactic positrons and propagation of low-energy positrons in the interstellar medium. Future MeV gamma-ray observations, such as COSI, are expected to confirm the reported features and shed further light on the nature of positrons in our Galaxy.

18 pages, 20 figures, accepted for publication in A&A

References in corpus (20)

• Giant Gamma-ray Bubbles from Fermi-LAT: AGN Activity or Bipolar Galactic Wind?
• Exciting Dark Matter and the INTEGRAL/SPI 511 keV signal
• The Spectrum and Morphology of the Fermi Bubbles
• The Fermi Bubbles: Giant, Multi-Billion-Year-Old Reservoirs of Galactic Center Cosmic Rays
• Detection of large-scale X-ray bubbles in the Milky Way halo
• Early 56Ni decay γ-rays from SN2014J suggest an unusual explosion
• SN2014J gamma-rays from the 56Ni decay chain
• XRBcats: Galactic Low Mass X-ray Binary Catalogue
• Microquasars as sources of positron annihilation radiation
• The Positron Puzzle
• Background modelling for $\gamma$-ray spectroscopy with INTEGRAL/SPI
• Imaging the 511 keV positron annihilation sky with COSI
• Reticulum II: Particle Dark Matter and Primordial Black Holes Limits
• A HARPS radial velocity search for planets in the Scorpius-Centaurus association. And its combination with the HARPS and SOPHIE young nearby stars (YNS) surveys
• A celestial gamma-ray foreground due to the albedo of small solar system bodies and a remote probe of the interstellar cosmic ray spectrum
• ${}^{26}$Al gamma rays from the Galaxy with INTEGRAL/SPI
• Gamma-Ray Light Curves and Spectra of Classical Novae
• The cosipy library: COSI's high-level analysis software
• Gamma-ray line emission from the Local Bubble
• Enhancing Compton telescope imaging with maximum a posteriori estimation: a modified Richardson-Lucy algorithm for the Compton Spectrometer and Imager

Cited by in corpus (1)

• Galactic Electron-Positron Annihilation Emission from X-Ray Binaries with Transient and Persistent Jets