Positron annihilation spectrum from the Galactic Centre region observed by SPI/INTEGRAL, revisited: annihilation in a cooling ISM?

Understand

We analyse SPI/INTEGRAL data on the 511 keV line from the Galactic Centre, accumulated over ~6 years of observations.

• We decompose the X-ray and soft gamma-ray emission of the central part of the Milky Way into a relatively compact "Bulge" and a more extended "Disk" components and report their spectral properties.
• The Bulge component shows a prominent 511 keV line and essentially no flux at 1.8 MeV, while the Disk component on the contrary contains a prominent 1.8 MeV line and a very weak annihilation line.
• We show that the spectral shape of the annihilation radiation (the narrow 511 keV line and the associated othro-positronium continuum) is surprisingly well described by a model of annihilation of hot positrons in a radiatively cooling interstellar medium (ISM).