Astronomy & Astrophysics

Free access article

A&A 441, 513-532 (2005)
DOI: 10.1051/0004-6361:20042063

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

J. Knödlseder¹, P. Jean¹, V. Lonjou¹, G. Weidenspointner¹, N. Guessoum², W. Gillard¹, G. Skinner¹, P. von Ballmoos¹, G. Vedrenne¹, J.-P. Roques¹, S. Schanne³, B. Teegarden⁴, V. Schönfelder⁵ and C. Winkler⁶

¹ Centre d'Étude Spatiale des Rayonnements, CNRS/UPS, BP 4346, 31028 Toulouse Cedex 4, France
e-mail: knodlseder@cesr.fr
² American University of Sharjah, College of Arts & Science, Physics Department, PO Box 26666, Sharjah, UAE
³ DSM/DAPNIA/SAp, CEA Saclay, 91191 Gif-sur-Yvette, France
⁴ Laboratory for High Energy Astrophysics, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁵ Max-Planck-Institut für Extraterrestrische Physik, Postfach 1603, 85740 Garching, Germany
⁶ ESA/ESTEC, Science Operations and Data Systems Division (SCI-SD), 2201 AZ Noordwijk, The Netherlands

(Received 24 September 2004 / Accepted 20 May 2005)

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 ~ $8\ensuremath{^\circ}$ (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 ~ $50\sigma$ , that of the galactic disk is ~ $4\sigma$ . 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 $(1.05 \pm 0.06) \times 10^$ ph cm^-2 s^-1 and $(0.7 \pm 0.4) \times 10^$ ph cm^-2 s^-1 respectively, corresponding to a bulge-to-disk flux ratio in the range 1-3. Assuming a positronium fraction of $f_{\rm p}=0.93$ this translates into annihilation rates of $(1.5 \pm 0.1) \times 10^$ s^-1and $(0.3 \pm 0.2) \times 10^$ 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 $3\sigma$ upper flux limit for this feature is $1.5 \times 10^$ ph cm^-2 s^-1. The disk emission can be attributed to the $\beta^+$ -decay of the radioactive species ²⁶ Al and ⁴⁴Ti. 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.

Key words: gamma rays: observations -- line: profiles -- Galaxy: center

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