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

Abstract

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.

Constraints on the kinematics of the ${^{44}}$Ti ejecta of Cassiopeia A from INTEGRAL/SPI

“…The use of the GEDSAT in our modelling is an hypothesis on the time variability of the real background and, as such, it could introduce some bias in the results. Yet, many analyses of SPI gamma-ray line observations have been performed under this prescription (see Knödlseder et al 2005;Wang et al 2007;Diehl et al 2006, for the annihilation, 60 Fe and 26 Al lines respectively) and produced successful results, so that adopting a similar strategy for the study of the 44 Ti lines seems justified; in addition we will ensure that the results are not affected by systematic effects through a thorough analysis of the residuals.…”

Cosmic Gamma-Ray Spectroscopy

“…INTEGRAL has imaged the positron annihilation gamma-rays across the sky in great detail, and confirmed the diffuse nature of annihilation across our Galaxy [70]. The scientific surprise of this emission had already been apparent in earlier results from the Compton Observatory [103], and was consolidated by SPI measurements: The annihilation emission predominantly arises in the inner Galaxy in an extended region of size 10 • .…”

“…The scientific surprise of this emission had already been apparent in earlier results from the Compton Observatory [103], and was consolidated by SPI measurements: The annihilation emission predominantly arises in the inner Galaxy in an extended region of size 10 • . By comparison, the disk of the Galaxy is much fainter, wit a bulge-to-disk intensity ratio of 1.4 from a total luminosity of 2 10 −3 ph cm −2 s −1 [70,144].…”

About cosmic gamma ray lines

“…6): the rate of supernovae of type core-collapse is about one supernova every 50 years [47], and radioactive lifetime of 26 Al is 1.04 mission years. Also 60 Fe (τ=3.8±0.05 My) supposedly is a diffuse gamma-ray line source in our Galaxy, and it seems that the lifetime of positrons between their injection and annihilation at rest seems of the order of 10 5 to 10 7 years, as emission in the 511 keV line apparently is diffuse in nature [48,49]. The spatial distribution of candidate sources therefore is only available indirectly, i.e.…”

“…INTEGRAL with its spectrometer SPI then allowed us to obtain a more detailed view of the positron annihilation gamma-ray emission [86,49,87,88,89,90,91]. The inner Galaxy with its bulge region was the dominant source, while the disk of the Galaxy was faint, contrasting expectations that positron sources would be related to active star forming regions where massive stars and binary interactions would conspire to create high-energy sources.…”