Cosmological γ-γ pair-production background

The origin of positrons is one of the unsolved puzzles in astrophysics as the majority of sources are still unidentified. The cosmic photon background (CPB) is the isotropic radiation spanning the entire electromagnetic spectrum. Interactions of the CPB with itself may pose a promising source of positrons and secondary emission. We aim to calculate the electron-positron pair-production rate from the γ-γ pair production of the CPB with itself for redshifts up to z = 10, and determine the annihilation spectrum as well as inverse Compton emission and bremsstrahlung. The CPB was decomposed into a sum of gray-body functions, of which each was evolved according to source type luminosity functions and redshift. We computed the pair-production rate by integrating the angle- and energy-dependent cross section over the evolving CPB. The pairs produced at each redshift were then propagated toward z=0, taking into account a cosmological, intergalactic, energy loss function. The photon emission was calculated per redshift and then line-of-sight-integrated toward a contribution of the current cosmic gamma-ray background (CGB). The resulting pair-production emissivity increases steeply from z=0 of about SI 2.0e-36 s^ -1 -3 to a peak of SI 1.8e-31 s^ -1 -3 at z=2.7, then it declines again. This yields a total cosmic pair-production rate on the order of 10^ 54 , up to redshift ten. The secondary emission of pairs experiencing inverse Compton scattering off the CPB results in a sizable contribution to the CGB. -1 The pairs from cosmological γ-γ absorption provide a minimum level of secondary emission, which needs to be taken into account for any CGB study. Especially in the range from 1,MeV to 1,GeV, this background can make up 10--20% of the total CGB emission and may substantially reduce the gap between MeV observations and models.