The lives and deaths of positrons in the interstellar medium

Abstract

Abstract. We reexamine in detail the various processes undergone by positrons in the interstellar medium (ISM) from their birth to their annihilation using the most recent results of positron interaction cross sections with atomic and molecular hydrogen, as well as helium. The positrons' lives are divided into two phases: the "in-flight" phase (between ≈1 MeV and tens of eV) and the thermal phase. The first phase is treated with a Monte Carlo simulation that allows us to determine the fraction of positrons that form positronium and annihilate as well as the characteristics of the annihilation emission as a function of the medium conditions. The second phase is treated with a binary reaction rate approach, with cross sections adopted from experimental measurement or theoretical calculations. An extensive search and update of the knowledge of positron processes was thus undertaken. New reaction rates and line widths have been obtained. We investigate the treatment of the complicated interactions between positrons and interstellar dust grains. Fully relevant data were not always available, but we were nonetheless able to reach satisfactory understanding of positron annihilation on grains, both qualitatively and quantitatively. All factors of the problem have been considered, including the grain size distribution and composition, the electric charge of the grains, the backscattering, positronium formation and ejection from the grain, the pick-off annihilation inside them and the partial destruction of dust in the hot regions of the ISM. New reaction rates and widths of the line resulting from the annihilation inside and outside of the grain have been obtained. The final results of our calculations (reaction rates and spectra) showed that dust is only important in the hot phase of the ISM, where it dominates all other processes. Combining the new calculations, we have constructed annihilation spectra for each phase of the ISM, considering various grain contents, as well as an overall combined spectrum for the ISM as a whole.

The Advanced Compton Telescope mission

“…In addition, the profile of the 511 keV line appears to be composed of two components, a narrow line (~1.5 keV wide), and a broader line (~6 keV) [45]. Collectively, the positronium fraction, line widths and narrow-tobroad line ratio are consistent with the expected signature of positron-electron annihilation occurring in a warm medium [35,45,16]. The intense bulge emission dominates the observations of these instruments.…”

Galactic annihilation emission from nucleosynthesis positrons

“…(for a complete review, see Guessoum et al 2005). The physical and chemical properties of the medium where the annihilation takes place set the dominant annihilation channels and as a result, each interstellar medium (ISM) phase has a typical annihilation spectrum.…”

Soft gamma-ray background and light dark matter annihilation

“…The resulting low-energy positrons are confined by the magnetic field of the bulge, where they are progressively slowed down by ionisation losses. A large fraction (0.93) forms positronium with ambient electrons and annihilate into two (25% of probability) or three γ photons [42]. Positronium formation plays therefore an important role because it decreases by a factor of about 3 the intensity of the 511 keV line.…”