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

Abstract

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° with a 2 σ uncertainty range covering 6°-18°. The 511 keV line flux in the bulge component amounts to 9.9-2.1+4.7 × 10 -4 ph cm-2 s-1. No evidence for a galactic disk component has been found so far; upper 2σ flux limits in the range (1.4-3.4) × 10-3 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.