Spectral analysis of the Galactic e⁺e^- annihilation emission

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-positronium continuum. The broad line is detected (significance 3.2σ) with a flux of (0.35 ± 0.11) × 10 −3 photons 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 positroniums that are formed by the charge exchange process of slowing down positrons with hydrogen atoms. The flux of the narrow line is (0.72 ± 0.12) × 10 −3 photons s −1 cm −2 and its width is 1.3 ± 0.4 keV FWHM. The measured orthopositronium continuum flux yields a fraction of positronium 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 (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.

Galactic annihilation emission from nucleosynthesis positrons

“…This scenario actually is motivated by the observation that the bulge and the disk have different ISM compositions: the bulge is largely dominated by the hot ionised phase, while the disk is mostly filled with neutral gas. Estimates for the filling factors of the warm neutral medium, warm ionised medium, and hot medium are respectively (0.2, 0.1, 0.7) for the bulge and (0.5, 0.3, 0.2) for the disk, while the molecular medium and cold neutral medium occupy comparatively negligible volumes (Jean et al 2006;Higdon et al 2009). From Sect.…”

“…The annihilation emission observed with SPI in the inner Galactic regions, the 511 keV line and the continuum below down to ∼400 keV, indicates that most positrons annihilate in the warm medium of the ISM (Churazov et al 2005;Jean et al 2006). Approximately one half annihilate in the warm ionised phase, through the formation of Ps by radiative recombination.…”

“…To annihilate, however, most will have to be slowed down to near-thermal energies, at least below ∼100 eV. This energy loss proceeds mostly through ionisation/excitation and Coulomb interactions in the interstellar gas, and it can take ∼10 5 yr for a typical interstellar density of 1 cm −3 (Jean et al 2006). Over that time positrons travel at relativistic speeds, which opens the possibility for large-scale transport (see below).…”

Cold positrons from decaying dark matter

“…The incoming cold positrons envisioned here would travel of order 200 pc in a HIM before annihilating (corresponding to d ann in Table 4 of Ref. [34]). A HIM region substantially larger than this would indeed deplete the incoming positron flux, thereby affecting the 511 keV fux.…”

Is There a Dark Matter Signal in the Galactic Positron Annihilation Radiation?

“…This corresponds to a positron annihilation rate in the Galactic bulge of ∼ 1 × 10 43 e + s −1 . Comparable annihilation radiation has been measured [2] from the Galactic disk and halo, which with their calculated [3,4] positron to 511 keV photon ratios give annihilation rates of ∼ 0.7 × 10 43 e + s −1 for each, assuming a Solar distance to the Galactic center of 8 kpc.…”