Positron annihilation spectrum from the Galactic Centre region observed by SPI/INTEGRAL revisited: annihilation in a cooling ISM?
Article first published online: 29 NOV 2010
© 2010 The Authors Monthly Notices of the Royal Astronomical Society © 2010 RAS
Monthly Notices of the Royal Astronomical Society
Volume 411, Issue 3, pages 1727–1743, March 2011
How to Cite
Churazov, E., Sazonov, S., Tsygankov, S., Sunyaev, R. and Varshalovich, D. (2011), Positron annihilation spectrum from the Galactic Centre region observed by SPI/INTEGRAL revisited: annihilation in a cooling ISM?. Monthly Notices of the Royal Astronomical Society, 411: 1727–1743. doi: 10.1111/j.1365-2966.2010.17804.x
- Issue published online: 18 FEB 2011
- Article first published online: 29 NOV 2010
- Accepted 2010 October 1. Received 2010 September 26; in original form 2009 December 31
- ISM: general;
- Galaxy: centre;
- gamma-rays: 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.