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

We analyse SPI/INTEGRAL data on the 511-keV line from the Galactic Centre, accumulated over ∼6 yr of observations. We decompose the X-ray and soft gamma-ray emission of the central part of the Milky Way into relatively compact 'bulge' and more extended 'disc' 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 disc 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 ortho-positronium continuum) is surprisingly well described by a model of annihilation of hot positrons in a radiatively cooling interstellar medium (ISM). The model assumes that positrons are initially injected into a hot (∼10⁶ K), volume-filling ISM, which is allowed to freely cool via radiative losses. The annihilation time in such a medium is longer than the cooling time for temperatures higher than a few 10⁴ K. Thus, most of the positrons annihilate only after the gas has cooled down to ∼10⁵ K, giving rise to annihilation emission characteristic of a warm, ionized ISM.