The sky distribution of 511 KeV positron annihilation line emission as measured with INTEGRAL/SPI

ESA/ESTEC, SCI-SD, 2201 AZ Noordwijk, The NetherlandsABSTRACTThe imaging spectrometer SPI on board ESA’s INTE-GRAL observatory provides us with an unprecedentedview of positron annihilation in our Galaxy. The firstsky maps in the 511 keV annihilation line and in thepositronium continuum from SPI showed a puzzling con-centration of annihilation radiation in the Galactic bulgeregion. By now, more than twice as many INTEGRALobservations are available, offering new clues to the ori-gin of Galactic positrons. We present the current statusof our analyses of this augmented data set. We now de-tect significant emission from outside the Galactic bulgeregion. The 511 keV line is clearly detected from theGalactic disk; in addition, there is a tantalizing hint atpossible halo-like emission. The available data do not yetpermit to discern whether the emission around the bulgeregion originates from a halo-like component or from adisk component that is very extended in latitude.Key words: Galactic 511 keV line emission; positron an-nihilation.1. INTRODUCTIONThe detection of 511 keV positron annihilation line emis-sion from the central region of our Galaxy was one ofthe early and important successes of gamma-ray astron-omy [17, 22]. Although positron annihilation gives riseto the strongest gamma-ray line signal from our Galaxy,three decades of dedicated observational and theoreticaleffort have fallen short of unveiling the origin of Galac-tic positrons. A large variety of potential astrophysicaland exotic positron sources have been proposed, includ-ing: interactionsofcosmic raysin the interstellarmedium[32]; pulsars [39] and millisecond pulsars [44]; the de-cay of radioactive nuclei expelled by supernovae [6], no-vae [7], Wolf-Rayet stars [29], or hypernovae/gamma-ray bursts [3]; compact objects harbouring black holesor neutron stars [33], low-mass X-ray binaries [30], andmicro-quasars [12]; gamma-ray bursts [23]; pair produc-tion by gamma rays from so-called Small Mass BlackHoles with X-ray photons from the supermassive blackhole at the center of our Galaxy, Sgr A