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

高能伽马射线与宇宙微波和红外波段的背景辐射碰撞，产生成对的电子和正电子，从而引发了在河外空间产生大量电子和伽马射线的级联过程。为了展望TeV γ射线天文学和研究宇宙中的超高能现象，计算了远距离源辐射γ射线的吸收和再生效应，次级γ射线的再生引起10-100 TeV能区γ射线通量的积累。然而，当一次伽马射线能谱硬到幂指数α为-2时，通量的增加因子最多为3左右，而当α等于2.5时，通量的增加因子小于10%。另一方面，能量高于10^{15} eV的电子（级联过程的次级产物）可以引起宇宙级联的更突出的效应。这种高能电子辐射同步加速器光子，其能量将进入与GeV伽马射线一样高的较低能带，可能提供极高能量伽马射线的证据。然而，这一过程取决于河外空间的磁场强度。因此，仍需进行更详细的调查。另一个可能出现的有趣的特征是电子能谱中的光谱断裂，这是由于电子与背景辐射光子碰撞时从汤姆逊散射到康普顿散射的过渡。级联过程导致入射伽马射线的相当大的角扩展，具有宽视场的望远镜预计将增加重要性，随着甚高能伽马射线天文学向更高能量区域和更远距离延伸。

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.