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 similar to 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 similar to 8 degrees. ( 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 similar to 50 sigma, that of the galactic disk is similar to 4 sigma . 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) x 10(-3) ph cm(-2) s(-1) and (0.7 +/- 0.4) x 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) x 10(43) s(-1) and (0.3 +/- 0.2) x 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 di. use emission, down to a typical flux limit of similar to 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 3 sigma upper flux limit for this feature is 1.5 x 10(-4) ph cm(-2) s(-1). The disk emission can be attributed to the beta(+)-decay of the radioactive species Al-26 and Ti-44. The bulge emission arises from a di. erent 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.