The all - sky distribution of 511 keV electron - positron annihilation emission ⋆

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 fp = 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 26 Al 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.