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.

arXiv comment18 pages, 20 figures, accepted for publication in A&A

References in corpus (42)

• HEALPix -- a Framework for High Resolution Discretization, and Fast Analysis of Data Distributed on the Sphere
• A Hipparcos census of the nearby OB associations
• Giant Gamma-ray Bubbles from Fermi-LAT: AGN Activity or Bipolar Galactic Wind?
• Radioactive 26Al and massive stars in the Galaxy
• Exciting Dark Matter and the INTEGRAL/SPI 511 keV signal
• The all-sky distribution of 511 keV electron-positron annihilation emission
• The Spectrum and Morphology of the Fermi Bubbles
• The 511 keV emission from positron annihilation in the Galaxy
• 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
• Cobalt-56 gamma-ray emission lines from the type Ia supernova 2014J
• SPI/INTEGRAL in-flight performance
• Gamma-ray spectroscopy of Positron Annihilation in the Milky Way
• Early 56Ni decay γ-rays from SN2014J suggest an unusual explosion
• Positron Escape from Type Ia Supernovae
• The lives and deaths of positrons in the interstellar medium
• A RIAF Interpretation for the Past Higher Activity of the Galactic Center Black Hole and the 511 keV Annihilation Emission
• SN2014J gamma-rays from the 56Ni decay chain
• On the morphology of the electron-positron annihilation emission as seen by SPI/INTEGRAL
• Positron transport in the interstellar medium
• Radioactive 26Al from the Scorpius-Centaurus Association
• XRBcats: Galactic Low Mass X-ray Binary Catalogue
• On the intensity and spatial morphology of the 511 keV emission in the Milky Way
• Surround & Squash: The Interstellar Medium around Scorpius Centaurus OB2
• INTEGRAL/SPI $\gamma$-ray line spectroscopy
• Revisiting INTEGRAL/SPI observations of 44Ti from Cassiopeia A
• Microquasars as sources of positron annihilation radiation
• Could electron-positron annihilation lines in the Galactic center result from pulsar winds?
• The Positron Puzzle
• Radiation environment along the INTEGRAL orbit measured with the IREM monitor
• Background modelling for $\gamma$-ray spectroscopy with INTEGRAL/SPI
• Imaging the 511 keV positron annihilation sky with COSI
• Positrons and 511 keV radiation as tracers of recent binary neutron star mergers
• 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 effect of positron-alkali metal atom interactions in the diffuse ISM
• 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

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