G. Skinner

SPI: The spectrometer aboard INTEGRAL

SPI is a high spectral resolution gamma-ray telescope on board the ESA mission INTEGRAL (International Gamma Ray Astrophysics Laboratory). It consists of an array of 19 closely packed germanium detectors surrounded by an active anticoincidence shield of BGO. The imaging capabilities of the instrument are obtained with a tungsten coded aperture mask located 1.7 m from the Ge array. The fully coded field-of-view is 16degrees, the partially coded field of view amounts to 31degrees, and the angular resolution is 2.5degrees. The energy range extends from 20 keV to 8 MeV with a typical energy resolution of 2.5 keV at 1.3 MeV. Here we present the general concept of the instrument followed by a brief description of each of the main subsystems. INTEGRAL was successfully launched in October 2002 and SPI is functioning extremely well.

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 ESAs 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.

Early SPI/INTEGRAL measurements of 511 keV line emission from the 4th quadrant of the Galaxy

We report the first measurements of the 511 keV line emission from the Galactic Centre (GC) region performed with the spectrometer SPI on the space observatory INTEGRAL (International Gamma-Ray Astrophysics Laboratory). Taking into account the range of spatial distribution models which are consistent with the data, we derive a flux of 9:9 +4:7 2:1 10 4 ph cm 2 s 1 and an intrinsic line width of 2:95 +0:45 0:51 keV (FWHM). The results are consistent with other high-spectroscopy measurements, though the width is found to be at the upper bound of previously reported values.We report the first measurements of the 511 keV line emission from the Galactic Centre (GC) region performed with the spectrometer SPI on the space observatory INTEGRAL (International Gamma-Ray Astrophysics Laboratory). Taking into account the range of spatial distribution models which are consistent with the data, we derive a flux of

SPI/INTEGRAL in-flight performance

9.9^{+4.7}_{-2.1} times 10^{-4}

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

ph cm

The Spiros imaging software for the Integral SPI spectrometer

^{-2}

INTEGRAL SPI limits on electron-positron annihilation radiation from the galactic plane

s

Monte Carlo simulations and generation of the SPI response

^{-1}

SPI instrumental background characteristics

and an intrinsic line width of

SPI-specific analysis method and software overview

2.95^{+0.45}_{-0.51}