A simple formula for the thermal pair annihilation line emissivity
  We introduce a simple and convenient fitting formula for the thermal annihilation line from pair plasmas in cosmic sources. The fitting formula is accurate to 0.04\% and is valid at all photon energies and temperatures of interest. The commonly used Gaussian line profile is not a good approximation for broader lines. 
Roland Svensson, Stefan Larsson and Juri Poutanen (Stockholm   Observatory)