Frontier of Energy and Distance of Very High Energy Gamma-Ray Astronomy

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Very energetic gamma rays collide with universal cosmic background radiation of microwave and infrared band to create pairs of electron and positrons, thus initiating a cascade process of creating a number of electrons and gamma rays in the extragalactic space. The effect of absorption and regeneration of gamma-rays which are radiated from far distant sources was calculated, in order to draw a view in near future of TeV Gamma Ray Astronomy and to study ultra high energy phenomena in the Universe.Regeneration of secondary gamma rays causes accumulation of gamma ray flux in the energy region around 10-100 TeV. However, the increasing factor of the flux is at most about 3 when energy spectrum of primary gamma rays is as hard as the power index alpha of -2, while the factor is less than 10% for in the case of alpha equal to 2.5. On the other hand, more prominent effect of the cosmic cascade can be caused by electrons, the secondary product of the cascade process, of energies higher than 10^{15} eV,. Such high energy electrons radiate synchrotron photons, of which energy would be into the lower energy band as high as GeV gamma rays, possibly to provide evidence of extremely high energy gamma rays. However, the process depends on the strength of magnetic field in the extragalactic space. Thus, there remain investigations to be done in more details. Another interesting feature which may appear is a spectral break in the energy spectrum of electrons, due to the transition from the Thomson scattering to Compton scattering in the collision of electrons with photons of the background radiation.The cascade process causes a considerable angular spread of incident gamma-rays, and the telescope with a wide field of view is expected to increase importance, as very high energy gamma ray astronomy extends into higher energy region and far distances.