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. 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 deg. The emission is equally well described by models that represent the stellar bulge or halo populations. 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 1.05e-3 ph cm-2 s-1 and 0.7e-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 0.93 this translates into annihilation rates of 1.5e43 s-1 and 3e42 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 1e-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 1.5e-4 ph cm-2 s-1. The disk emission can be attributed to the beta+ decay of the radioactive species 26Al 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.Comment: accepted for publication in A&

Early SPI/INTEGRAL contraints on the morphology of the 511 keV line emission in the 4th galactic quadrant

We provide first constraints on the morphology of the 511 keV line emission from the galactic centre region on basis of data taken with the spectrometer SPI on the INTEGRAL gamma-ray observatory. The data suggest an azimuthally symmetric galactic bulge component with FWHM of ~9 deg with a 2 sigma uncertainty range covering 6-18 deg. The 511 keV line flux in the bulge component amounts to (9.9+4.7-2.1) 10e-4 ph cm-2 s-1. No evidence for a galactic disk component has been found so far; upper 2 sigma flux limits in the range (1.4-3.4) 10e-3 ph cm-2 s-1 have been obtained that depend on the assumed disk morphology. These limits correspond to lower limits on the bulge-to-disk ratio of 0.3-0.6.Comment: 5 pages, 3 figures, accepted for publication in A&

SPI observations of positron annihilation radiation from the 4th galactic quadrant: sky distribution

During its first year in orbit the INTEGRAL observatory performed deep exposures of the Galactic Center region and scanning observations of the Galactic plane. We report on the status of our analysis of the positron annihilation radiation from the 4th Galactic quadrant with the spectrometer SPI, focusing on the sky distribution of the 511 keV line emission. The analysis methods are described; current constraints and limits on the Galactic bulge emission and the bulge-to-disk ratio are presented.Comment: 4 pages, 2 figures, accepted for publication in the proceedings of the 5th INTEGRAL worksho

INTEGRAL/IBIS search for e-e+ annihilation radiation from the Galactic Center Region

Electron-positron annihilation radiation from the Galactic Center region has been detected since the seventies, but its astrophysical origin is still a topic of a scientific debate. We have analyzed data of the gamma-ray imager IBIS/ISGRI onboard of ESA's INTEGRAL platform in the e$^{-}$e$^{+}$ line. During the first year of the missions Galactic Center Deep Exposure no evidence for point sources at 511 keV has been found in the ISGRI data; the $2 \sigma$ upper limit for resolved single point sources is estimated to be $1.6\times 10^{-4} ph cm^{-2} s^{-1}$.Comment: 6 pages, 3 figures; Cospar 2004. To be published in: Advances in Space Researc

INTEGRAL/SPI Limits on Electron-Positron Annihilation Radiation from the Galactic Plane

The center of our Galaxy is a known strong source of electron-positron 511-keV annihilation radiation. Thus far, however, there have been no reliable detections of annihilation radiation outside of the central radian of our Galaxy. One of the primary objectives of the INTEGRAL (INTErnational Gamma-RAy Astrophysics Laboratory) mission, launched in Oct. 2002, is the detailed study of this radiation. The Spectrometer on INTEGRAL (SPI) is a high resolution coded-aperture gamma-ray telescope with an unprecedented combination of sensitivity, angular resolution and energy resolution. We report results from the first 10 months of observation. During this period a significant fraction of the observing time was spent in or near the Galactic Plane. No positive annihilation flux was detected outside of the central region (|l| > 40 deg) of our Galaxy. In this paper we describe the observations and data analysis methods and give limits on the 511-keV flux.Comment: Accepted for publication in the Astrophysical Journal. 13 pages, 3 figure

The sky distribution of positronium annihilation continuum emission measured with SPI/INTEGRAL

We present a measurement of the sky distribution of positronium (Ps) annihilation continuum emission obtained with the SPI spectrometer on board ESA's INTEGRAL observatory. The only sky region from which significant Ps continuum emission is detected is the Galactic bulge. The Ps continuum emission is circularly symmetric about the Galactic centre, with an extension of about 8 deg FWHM. Within measurement uncertainties, the sky distribution of the Ps continuum emission is consistent with that found by us for the 511 keV electron-positron annihilation line using SPI. Assuming that 511 keV line and Ps continuum emission follow the same spatial distribution, we derive a Ps fraction of 0.92 +/- 0.09. These results strengthen our conclusions regarding the origin of positrons in our Galaxy based on observations of the 511 keV line. In particular, they suggest that the main source of Galactic positrons is associated with an old stellar population, such as Type Ia supernovae, classical novae, or low-mass X-ray binaries. Light dark matter is a possible alternative source of positrons.Comment: accepted for publication by A&

INTEGRAL/SPI Limits on Electron-Positron Annihilation Radiation from the Galactic Plane

The center of our Galaxy is a known strong source of electron-positron 511 keV annihilation radiation. Thus far, however, there have been no reliable detection of annihilation radiation outside of the central radian of our Galaxy. One of the primary objectives of the INTEGRAL (International Gamma-Ray Astrophysics Laboratory mission, resolution, coded-apeture gamma-ray telescope with an unprecedented combination of sensitivity, angular resolution, and energy resolution. We resport results from the first 10 months of observation. During this period a significant fraction of the observing time was spent in or near the Galactic plan. No positive annihilation flux was detected outside of the central regin ( l \u3e 40°) of our Galaxy. In this paper we describe observation and data analysis method and give limits on the 511 keV flu