Spectral analysis of the Galactic e+e- annihilation emission

TL;DR

This study analyzes the Galactic Centre e$^{+}$e$^{-}$ annihilation spectrum from INTEGRAL/SPI to identify the ISM phase where annihilation occurs and to constrain positron sources. It employs two spectral models, an independent Gaussian-based approach and an ISM-phase model informed by Guessoum, Jean & Gillard 2005, to fit the data with a $511$ keV line, a broad component from positronium in flight, and an ortho-positronium continuum, finding a consistent interpretation that annihilation is dominated by the warm neutral and warm ionized phases ($f_{wn}\approx0.49$, $f_{wi}\approx0.51$) and a total positronium fraction of $f_{Ps}\approx0.935$. The broad line width of about $5.36$ keV and the narrow line width of about $1.32$ keV align with expectations for positronium formation in flight in warm gas and thermalized annihilation, respectively, while no strong evidence for hot gas or molecular cloud contributions is found. These results imply diffuse, sub-MeV positron sources with propagation largely confined to warm ISM, providing constraints on positron transport, the ISM content in the bulge, and the nature of positron sources in the old stellar population.

Abstract

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-Ps continuum. The broad line is detected with a flux of (0.35+/-0.11)e-3 s-1 cm-2. The measured width of 5.4+/-1.2 keV FWHM is in agreement with the expected broadening of 511 keV photons emitted in the annihilation of Ps that are formed by the charge exchange process of slowing down positrons with H atoms. The flux of the narrow line is (0.72+/-0.12)e-3 s-1 cm-2 and its width is 1.3+/-0.4 keV FWHM. The measured ortho-Ps continuum flux yields a fraction of Ps of (96.7+/-2.2)%. 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(+2,-23)% of the annihilation emission comes from the warm neutral phase and 51(+3,-2)% 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 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.

Figures (6)

• Figure 1: Spectrum obtained by model fitting (see text). The instrumental background spectrum is shown for comparison.
• Figure 2: Time series of residuals in number of sigma unit obtained by subtracting the background plus sky models to the data. Two energy bands are shown for comparison: (a) 435--443 keV and (b) 526--534 keV. The date are in IJD (see text).
• Figure 3: Fit of the spectrum measured by SPI with contributions from a Gaussian line, an ortho-positronium continuum and a power-law Galactic continuum. A single Gaussian does not give a good fit to the flux measured in the wings of the line.
• Figure 4: Fit of the spectrum measured by SPI with narrow and broad Gaussian lines, an ortho-positronium continuum and a power-law Galactic continuum (constant slope of -1.75). Note that the asymmetric shape of the lines is due to the convolution of the Gaussian with the spectral response of SPI (Compton continuum and degradation).
• Figure 5: Best fit of the spectrum measured by SPI with the warm components of the ISM and the Galactic continuum. Contributions from the molecular, cold and hot components are not needed to explain the data.