γ -ray astronomy: γ -ray spectral lines
 
γ -rays may be generated by a variety of mechanisms such as matter–antimatter annihilation, radioactive decay, energetic particle collisions, synchrotron radiation (high-energy electrons being deviated by interstellar particles), Bremsstrahlung (high-energy electrons being deviated by a magnetic field) and by hot plasmas. Some other mechanisms involve the production of γ -rays with discrete energies so allowing positive identification of the process. The most famous γ -ray line emission is that at 0.511 MeV resulting from an e⁻e⁺ collision at

Figure1. The generation of ˇCerenkov radiation.
rest, denoted by
e⁺ + e⁻ → γ + γ.
This line has been detected by the HEAO-3 satellite in the direction of the galactic centre. A possible explanation involves the production of positrons as secondaries in the interactions of cosmic rays with the interstellar gas—subsequent annihilation produces the 0.511 MeV line. However, the strength of the line appears to be too great in relation to the cosmic-ray intensity at the galactic centre. An alternative explanation involves the accretion of gas by a black hole with a mass several million times that of the Sun. Hot spots within the disc emit e⁺e⁻ pairs. The positrons are then annihilated to produce the 0.511 MeV γ -ray line.