Abstract. We present a map of 511 keV electron-positron annihilation emission, based on data accumulated with the SPI spectrometer aboard ESA's INTEGRAL gamma-ray observatory, that covers approximately ∼95% of the celestial sphere. Within the exposed sky area, 511 keV line emission is significantly detected towards the galactic bulge region and, at a very low level, from the galactic disk. The bulge emission is highly symmetric and is centred on the galactic centre with an extension of ∼8• (FWHM). The emission is equally well described by models that represent the stellar bulge or halo populations. The detection significance of the bulge emission is ∼50σ, that of the galactic disk is ∼4σ. The disk morphology is only weakly constrained by the present data, being compatible with both the distribution of young and old stellar populations. The 511 keV line flux from the bulge and disk components is (1.05 ± 0.06) × 10 −3 ph cm −2 s −1 and (0.7 ± 0.4) × 10 −3 ph cm −2 s −1 , respectively, corresponding to a bulge-to-disk flux ratio in the range 1−3. Assuming a positronium fraction of f p = 0.93 this translates into annihilation rates of (1.5 ± 0.1) × 10 43 s −1 and (0.3 ± 0.2) × 10 43 s −1 , respectively. The ratio of the bulge luminosity to that of the disk is in the range 3−9. We find no evidence for a point-like source in addition to the diffuse emission, down to a typical flux limit of ∼10 −4 ph cm −2 s −1 . We also find no evidence for the positive latitude enhancement that has been reported from OSSE measurements; our 3σ upper flux limit for this feature is 1.5 × 10 −4 ph cm −2 s −1 . The disk emission can be attributed to the β + -decay of the radioactive species 26Al and 44 Ti. The bulge emission arises from a different source which has only a weak or no disk component. We suggest that Type Ia supernovae and/or low-mass X-ray binaries are the prime candidates for the source of the galactic bulge positrons. Light dark matter annihilation could also explain the observed 511 keV bulge emission characteristics.

The first gamma-ray line originating from outside the solar system that was ever detected is the 511 keV emission from positron annihilation in the Galaxy. Despite 30 years of intense theoretical and observational investigation, the main sources of positrons have not been identified up to now. Observations in the 1990's with OSSE/CGRO showed that the emission is strongly concentrated towards the Galactic bulge. In the 2000's, the SPI instrument aboard ESA's INTEGRAL γ-ray observatory allowed scientists to measure that emission across the entire Galaxy, revealing that the bulge/disk luminosity ratio is larger than observed in any other wavelength. This mapping prompted a number of novel explanations, including rather "exotic" ones (e.g. dark matter annihilation). However, conventional astrophysical sources, like type Ia supernovae, microquasars or X-ray binaries, are still plausible candidates for a large fraction of the observed total 511 keV emission of the bulge. A closer study of the subject reveals new layers of complexity, since positrons may propagate far away from their production sites, making it difficult to infer the underlying source distribution from the observed map of 511 keV emission. However, contrary to the rather well understood propagation of high energy (>GeV) particles of Galactic cosmic rays, understanding the propagation of low energy (∼MeV) positrons in the turbulent, magnetized interstellar medium, still remains a formidable challenge. We review the spectral and imaging properties of the observed 511 keV emission and we critically discuss candidate positron sources and models of positron propagation in the Galaxy.

We present a spectral analysis of the e + e − annihilation emission from the Galactic Centre region based on the first year of measurements made with the spectrometer SPI of the INTEGRAL mission. We have found that the annihilation spectrum can be modelled by the sum of a narrow and a broad 511 keV line plus an ortho-positronium continuum. The broad line is detected (significance 3.2σ) with a flux of (0.35 ± 0.11) × 10 −3 photons s −1 cm −2 . The measured width of 5.4 ± 1.2 keV FWHM is in agreement with the expected broadening of 511 keV photons emitted in the annihilation of positroniums that are formed by the charge exchange process of slowing down positrons with hydrogen atoms. The flux of the narrow line is (0.72 ± 0.12) × 10 −3 photons s −1 cm −2 and its width is 1.3 ± 0.4 keV FWHM. The measured orthopositronium continuum flux yields a fraction of positronium of (96.7 ± 2.2)%.To derive in what phase of the interstellar medium positrons annihilate, we have fitted annihilation models calculated for each phase to the data. We have found that 49 +2 −23 % of the annihilation emission comes from the warm neutral phase and 51 +3 −2 % from the warm ionized phase. While we may not exclude that less than 23% of the emission might come from cold gas, we have constrained the fraction of annihilation emission from molecular clouds and hot gas to be less than 8% and 0.5%, respectively. We have compared our knowledge of the interstellar medium in the bulge (size, density, and filling factor of each phase) and the propagation of positrons with our results and found that they are in good agreement if the sources are diffusively distributed and if the initial kinetic energy of positrons is lower than a few MeV. Despite its large filling factor, the lack of annihilation emission from the hot gas is due to its low density, which allows positrons to escape this phase.

Abstract. We reexamine in detail the various processes undergone by positrons in the interstellar medium (ISM) from their birth to their annihilation using the most recent results of positron interaction cross sections with atomic and molecular hydrogen, as well as helium. The positrons' lives are divided into two phases: the "in-flight" phase (between ≈1 MeV and tens of eV) and the thermal phase. The first phase is treated with a Monte Carlo simulation that allows us to determine the fraction of positrons that form positronium and annihilate as well as the characteristics of the annihilation emission as a function of the medium conditions. The second phase is treated with a binary reaction rate approach, with cross sections adopted from experimental measurement or theoretical calculations. An extensive search and update of the knowledge of positron processes was thus undertaken. New reaction rates and line widths have been obtained. We investigate the treatment of the complicated interactions between positrons and interstellar dust grains. Fully relevant data were not always available, but we were nonetheless able to reach satisfactory understanding of positron annihilation on grains, both qualitatively and quantitatively. All factors of the problem have been considered, including the grain size distribution and composition, the electric charge of the grains, the backscattering, positronium formation and ejection from the grain, the pick-off annihilation inside them and the partial destruction of dust in the hot regions of the ISM. New reaction rates and widths of the line resulting from the annihilation inside and outside of the grain have been obtained. The final results of our calculations (reaction rates and spectra) showed that dust is only important in the hot phase of the ISM, where it dominates all other processes. Combining the new calculations, we have constructed annihilation spectra for each phase of the ISM, considering various grain contents, as well as an overall combined spectrum for the ISM as a whole.

Abstract. We present a measurement of the sky distribution of positronium (Ps) annihilation continuum emission obtained with the SPI spectrometer on board ESA's INTEGRAL observatory. The only sky region from which significant Ps continuum emission is detected is the Galactic bulge. The Ps continuum emission is circularly symmetric about the Galactic centre, with an extension of about 8• FWHM. Within measurement uncertainties, the sky distribution of the Ps continuum emission is consistent with that found by us for the 511 keV electronpositron annihilation line using SPI. Assuming that 511 keV line and Ps continuum emission follow the same spatial distribution, we derive a Ps fraction of 0.92 ± 0.09. These results strengthen our conclusions regarding the origin of positrons in our Galaxy based on observations of the 511 keV line. In particular, they suggest that the main source of Galactic positrons is associated with an old stellar population, such as Type Ia supernovae, classical novae, or low-mass X-ray binaries. Light dark matter is a possible alternative source of positrons.