Positron astrophysics and positron states in the low-density Galactic medium

Properties of positron states in the low-density Galactic medium are briefly considered and analyzed. It is shown that the main positron states in this medium, in addition to fast positrons, are: 'thermalized' positrons, positronium atoms, and positron-bound states on hydrogen atoms. Positron annihilation from positronium atoms and bound states results in a sharp narrowing of the annihilation line Doppler broadening curves, in agreement with the available experimental data.