Extragalactic electromagnetic cascades and cascade gamma-ray emission in magnetic fields of various strength

We discuss the magnetic field influence on diffuse gamma-ray emission from extragalactic electromagnetic cascades initiated by ultra-high energy cosmic rays. Regions in space vary considerably in field strength: it is possibly of 10^(-12) G and lower in voids, of ~10^(-6) G inside galaxies, galactic clusters and groups, of ~10^(-7) G around them, and of ~ 10^(-8)-10^(-9) G in filaments. Structures having fields higher than in voids occupy comparatively small fraction of the Universe, so they affect weakly on cascade emission. Still knowledge of this influence may be relevant studying large-scale component of the extragalactic magnetic field and to the search for exotic particles, as in the latter case contribution of all components to extragalactic gamma-ray background should be known, one of which is cascade emission. To study magnetic field effect we simulate particle propagation in homogeneous magnetic field of ~10^(-6), 10^(-9), and 10^(-12) G and lower. It is found that in fields of ~10^(-9) G and lower the spectra of diffuse cascade gamma-rays at energies E<=10^17 eV coincide. Thus no specific models of EGMF are required to study contribution of cascade emission in the extragalactic gamma-ray background at E<=10^17 eV. In the case of uniform field of 10^(-6) G (which seems to be unrealistic), this inference is valid in the energy range of ~10^7-10^9 eV. Results obtained can be also used studying large-scale component of the extragalactic magnetic field.Comment: 13 pages, 3 figures, accepted in Advances in Space Research, avaliable online 26 Sept 202