The annihilation of galactic positrons

Abstract

We have studied the annihilation of galactic positrons in order to evaluate the probabilities of various channels of annihilation and to calculate the spectrum of the resulting radiations The narrow width (FWW < 3 " 2 keV) of the 0.511 MeV line observed from the Galactic Center ( Leventhal et al. 1978) implies that a large fraction of positrons should annihilate in a medium of temperature less than 10 5K and ionization fraction greater than 0905, HIl regions at the Galactic Center could be possible sites of annihilation. I I. INTRODUCTION Leventhal, NcCallum and Stang (1978) have recently reported observation of positron annihilation radiation from the Galactic Center using a balloon borne germanium detector. The observed line is at 510.7 ±0.5 keV, and its full width at half maximum (FWHM) is less than 3.2 keV. There is also some evidence for the three--photon continuum from triplet positronium annihilation. The 0.511 MeV line was previously seen from the solar flares of 1972, August 4 and 7 (Chupp, Forrest and Suri 1975), but because the lower energy resolution of the Nal detector used, only an upper limit of about 40 keV could be set on the line width from this observation. Depending on the temperature and density of the ambient medium, positrons and electrons can either annihilate directly or form positronium. The importance of positronium formation in the interstellar medium was pointed out by Steigman (1968), by Stecker (1969), and by Leventhal (1973), and positron annihilation in solar flares, both direct and via positronium, has been treated by Crannell et al. (1976). Positronium can form either in the singlet state which annihilates into two 0.511 MeV photons, or in the triplet state which decays by 3 photon annihilation. Positronium is formed by both radiative recombination with free electrons and charge exchange with neutral hydrogen (atomic or molecular). Charge exchange with heavier ions is much less important than these processes (Crannell et al. 1976). Once formed in a particular spin state, positronium in the interstellar medium annihilates from the same state, because the lifetimes of both singlet and triplet positronium (10 -10 sec and 10 -7 sec, respectively) are much shorter than typical collision times with interstellar gas.

X- and Gamma-Ray Line Emission Processes

“…Positrons produced by other processes generally have higher initial energies. Having lost the bulk of its kinetic energy, a positron can either form a positronium (Ps) atom in flight by charge exchange with an atom or a molecule (at < ∼ 50 eV in neutral H or H 2 , Bussard et al 1979) or become thermalized with the free electrons. Thermal positrons either annihilate directly with free or bound electrons or form Ps atoms through radiative recombination and charge exchange (Fig.…”

Positron-Electron Annihilation Radiation from the Galactic Center

“…The observed line width requires (Bussard, Ramaty and Drachman 1979) that this gas be at least partially ionized (n e > O.ln). If the gas were neutral, the line width would be larger than observed because it would be Doppler broadened, not by the thermal motion of the gas, but by the velocity of energetic positrons forming positronium in flight by charge exchange with neutral hydrogen.…”

Cosmic Gamma-Ray Spectroscopy

“…Interactions of low-energy cosmic rays ('LECR') appear to be closest to detection thresholds of current telescopes, while characteristic lines from nuclear transitions in accreting compact stars and their plasma jets are beyond reach at present. (3) Annihilation of particles with their anti-particles, such as electron-positron annihilation, which results in a characteristic line at 511 keV energy from twophoton annihilation, and a characteristic spectral feature extending from 511 keV down into the 100 keV region, which originates from 3-photon annihilation from the triplet state of the positronium atom which is formed on the mostlikely annihilation pathways [12]. This characteristic emission has been mapped to occur in an extended region throughout the inner parts of our Galaxy, has been reported also from a few transient events in compact sources, and has been observed in great detail from solar flares [99].…”