Journal Article

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

E. Churazov, S. Sazonov, S. Tsygankov, R. Sunyaev and D. Varshalovich

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 411, issue 3, pages 1727-1743
Published in print March 2011 | ISSN: 0035-8711
Published online February 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2010.17804.x
Positron annihilation spectrum from the Galactic Centre region observed by SPI/INTEGRAL revisited: annihilation in a cooling ISM?

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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.

Keywords: ISM: general; Galaxy: centre; gamma-rays: ISM

Journal Article.  13090 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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