Issue |
A&A
Volume 445, Number 2, January II 2006
|
|
---|---|---|
Page(s) | 579 - 589 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20053765 | |
Published online | 16 December 2005 |
Spectral analysis of the Galactic e+e- annihilation emission
1
CESR, CNRS/Université Paul Sabatier Toulouse 3, BP 4346, 31028 Toulouse Cedex 4, France e-mail: jean@cesr.fr
2
American University of Sharjah, College of Arts & Sciences, Physics Department, PO Box 26666, Sharjah, UAE
3
LATT, CNRS/OMP, 31000 Toulouse, France
Received:
5
July
2005
Accepted:
25
August
2005
We present a spectral analysis of the e+e- annihilation emission
from the Galactic Centre region based on the first year of measurements
made with the spectrometer SPI of the INTEGRAL mission.
We have found that the annihilation spectrum can be modelled by the
sum of a narrow and a broad 511 keV line plus an ortho-positronium
continuum. The broad line is detected (significance 3.2σ)
with a flux of ()
10-3 photons s-1 cm-2.
The measured width of
keV FWHM is in agreement with the
expected broadening of 511 keV photons emitted in the annihilation of
positroniums that are formed by the charge exchange process of
slowing down positrons with hydrogen atoms. The flux of the narrow line
is (
)
10-3 photons s-1 cm-2 and its width
is
keV FWHM. The measured ortho-positronium continuum
flux yields a fraction of positronium of (
)%.
To derive in what phase of the interstellar medium positrons
annihilate, we have fitted annihilation models calculated for
each phase to the data.
We have found that 49
% of the annihilation emission
comes from the warm neutral phase and 51
% from the warm
ionized phase. While we may not exclude that less than 23% of the emission
might come from cold gas, we have constrained the fraction of
annihilation emission from molecular clouds and hot gas to be less
than 8% and 0.5%, respectively.
We have compared our knowledge of the interstellar medium in the
bulge (size, density, and filling factor of each phase) and the
propagation of positrons with our results and found that they are in good
agreement if the sources are diffusively distributed and if the initial
kinetic energy of positrons is lower than a few MeV. Despite its large
filling factor, the lack of annihilation emission from the hot gas is
due to its low density, which allows positrons to escape this phase.
Key words: gamma rays: observations / line: formation / line: profiles / ISM: general
© ESO, 2005
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