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The origin of the positrons responsible for the 511 keV annihilation radiation is a 40 year old mystery and one which has been a major objective for INTEGRAL since its inception. The signal being sought is small compared with the strong instrumental background line at 511 keV and accurate modelling of that background line is a crucial aspect of the data analysis. Apparently inconsistent results have been reported using different approaches to background modelling. We report on an analysis of all of the data available to us, including data obtained in a new ‘pingpong’ mode and on statistical assessments of different approaches to background subtraction. We conclude that there is indeed evidence for an asymmetry that has been reported between the emission from negative and positive longitudes but that this asymmetry is not best described by a simple imbalance in the disk emission, nor by an offset of the galactic bulge emission, but by a combination of the two. The apparent inconsistencies probably arise as a result of a combination of the differences between the background modelling approaches and a structure that is more complex than either of these two models suggests.

The Galactic positron annihilation radiation: an update based on recent SPI observations