  Recent observations of the 511 keV positron-electron annihilation line in the
Galactic centre region by the INTEGRAL/SPI spectrometer have stirred up new
speculations about the origin of the large corresponding positron injection
rate. Beyond astrophysical candidates, new mechanisms have been put forward. We
focus on the annihilation of light dark matter particles and review the various
gamma-ray radiation components related to such a source of mono-energetic
positrons in addition to the 511 keV line itself. We study the influence of the
degree of ionisation of the bulge on this radiation, and its possible effects
on the observational constraints on the mass of the hypothetical light dark
matter particle or the injection energy of a mono-energetic source of positrons
in general.
