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

Abstract

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 (∼10 6 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 10 4 K. Thus, most of the positrons annihilate only after the gas has cooled down to ∼10 5 K, giving rise to annihilation emission characteristic of a warm, ionized ISM.

Gamma-ray spectroscopy of positron annihilation in the Milky Way

“…the region where the annihilation takes place Churazov et al 2005Churazov et al , 2011, the propagation distances are smaller. Thus, irrespective of whether the positrons are produced in young massive stars near the clouds, or, more diffusely, on kpc scales in SNe Ia of much older, accreting white dwarves, they will annihilate predominantly in these warm phases.…”

Inversion of the decay of the cyclotron line energy in Hercules X-1

“…The two data points shown in Fig. 1 are therefore from SPI (see also Ferrigno et al 2016), screened and reduced according to procedures described by Churazov et al (2011).…”

Diffuse emission study with INTEGRAL/SPI: Results after 10 years of observations

“…Quantitatively, the flux extracted through the model fitting method for the inner Galaxy |l| ≤ 30 • ,|b| ≤ 10 • is found to be around 3.3× 10 −4 ph cm −2 s −1 while it does not depend much on the sky model (particularly if we restrict ourselves to the preferred ones). Similarly, [6] using a light-bucket" method obtain a flux 4 × 10 −4 ph cm −2 s −1 for the region delimited by |l| ≤ 40 • ,|b| ≤ 40 • . For the same region, we obtain a flux of ∼ 3.9 × 10 −4 ph cm −2 s −1 with the most probable spatial morphologies.…”

“…Thanks to its imaging capabilities, the telescope is able to disentangle the spatial morphology and the spectral shape of the various emission components; each one is linked to specific physical processes. Using SPI data, only the morphologies study of the diffuse continuum ( [3]) and the 511 keV electron-positron line ( [4,5,6]) have been performed. However, to date, only the COMPTEL Compton telescope aboard the Compton Gamma Ray Observatory has mapped the 26 Al during its nine years survey.…”