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The sky distribution of 511 keV positron annihilation line emission as measured with INTEGRAL/SPI

G. Weidenspointner, J. Knoedlseder, P. Jean, G. K. Skinner, J. -P. Roques, G. Vedrenne, P. Milne, B. J. Teegarden, R. Diehl, A. Strong, S. Schanne, B. Cordier, C. Winkler

TL;DR

This work investigates the large-scale distribution of Galactic positrons through the 511 keV annihilation line using INTEGRAL/SPI data collected over more than two years. It employs imaging via Multi-Resolution Expectation Maximization (MREM) and a multi-component maximum-likelihood sky model that combines bulge, disk, and halo geometries, including nested shells and stellar-disk templates. The analysis reveals significant 511 keV emission from the Galactic disk and suggests possible halo-like extended emission, though the interpretation is limited by nonuniform sky exposure and degeneracies between disk and halo components. The results advance the understanding of the spatial distribution of Galactic positrons, highlighting a strong bulge component with detectable disk emission and motivating continued observations to resolve the halo versus latitude-extended disk question.

Abstract

The imaging spectrometer SPI on board ESA's INTEGRAL observatory provides us with an unprecedented view of positron annihilation in our Galaxy. The first sky maps in the 511 keV annihilation line and in the positronium continuum from SPI showed a puzzling concentration of annihilation radiation in the Galactic bulge region. By now, more than twice as many INTEGRAL observations are available, offering new clues to the origin of Galactic positrons. We present the current status of our analyses of this augmented data set. We now detect significant emission from outside the Galactic bulge region. The 511 keV line is clearly detected from the Galactic disk; in addition, there is a tantalizing hint at possible halo-like emission. The available data do not yet permit to discern whether the emission around the bulge region originates from a halo-like component or from a disk component that is very extended in latitude.

The sky distribution of 511 keV positron annihilation line emission as measured with INTEGRAL/SPI

TL;DR

This work investigates the large-scale distribution of Galactic positrons through the 511 keV annihilation line using INTEGRAL/SPI data collected over more than two years. It employs imaging via Multi-Resolution Expectation Maximization (MREM) and a multi-component maximum-likelihood sky model that combines bulge, disk, and halo geometries, including nested shells and stellar-disk templates. The analysis reveals significant 511 keV emission from the Galactic disk and suggests possible halo-like extended emission, though the interpretation is limited by nonuniform sky exposure and degeneracies between disk and halo components. The results advance the understanding of the spatial distribution of Galactic positrons, highlighting a strong bulge component with detectable disk emission and motivating continued observations to resolve the halo versus latitude-extended disk question.

Abstract

The imaging spectrometer SPI on board ESA's INTEGRAL observatory provides us with an unprecedented view of positron annihilation in our Galaxy. The first sky maps in the 511 keV annihilation line and in the positronium continuum from SPI showed a puzzling concentration of annihilation radiation in the Galactic bulge region. By now, more than twice as many INTEGRAL observations are available, offering new clues to the origin of Galactic positrons. We present the current status of our analyses of this augmented data set. We now detect significant emission from outside the Galactic bulge region. The 511 keV line is clearly detected from the Galactic disk; in addition, there is a tantalizing hint at possible halo-like emission. The available data do not yet permit to discern whether the emission around the bulge region originates from a halo-like component or from a disk component that is very extended in latitude.

Paper Structure

This paper contains 6 sections, 3 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Instrument and data analysis
  3. Results
  4. Imaging
  5. Model Fitting
  6. Summary and discussion

Figures (3)

  • Figure 1: Top panel: the exposure to the sky of the data set used in this analysis. Contour levels are at $1$, $2$, $3$, and $4 \times 10^7$ cm$^2$ s. Bottom panel: the exposure to the sky after the first year of the mission, depicted with the same color coding than above. Contour levels are at $1$ and $2 \times 10^7$ cm$^2$ s.
  • Figure 2: An MREM sky map of the 511 keV positron annihilation line emission. The contours indicate intensity levels of $10^{-2}$, $10^{-3}$, and $10^{-4}$ ph cm$^{-2}$ s$^{-1}$ sr$^{-1}$. Details are given in the text.
  • Figure 3: Left panel: the variation of the maximum likelihood ratio $\lambda$ when fitting the data with the Robin03 young disk plus an ellipsoidal model (FWHM $\Gamma_l = 6.5^\circ$ and $\Gamma_b = 6.0^\circ$) of the bulge emission. Center panel: as left panel, except that the bulge emission is modelled by a combination of 0--0.5 kpc and 0.5--1.5 kpc homogeneous shells. Right panel: as left and center panels, except that the emission from the central region of the Galaxy is described by the stellar halo model of Robin03. The dashed lines indicate decreases of $\lambda$ by 1 and 4, corresponding to 1 and 2$\sigma$ confidence levels for 1 degree of freedom.