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

高能伽马射线与宇宙背景辐射的微波和红外波段发生碰撞，产生电子和正电子对，从而在河外空间产生大量电子和伽马射线的级联过程。计算了遥远源辐射的伽玛射线的吸收和再生效应，为将来的TeV伽玛射线天文学和宇宙中超高能量现象的研究画上了一个远景。二次伽马射线的再生导致伽马射线通量在10-100 TeV左右的能量区域积累。然而，当一次伽马射线能谱与幂指数α = -2相当时，通量的增加因子最多为3左右，而当α = 2.5时，通量的增加因子小于10%。另一方面，能量高于10^{15}eV的级联过程的次级产物电子可能引起宇宙级联的更突出的效应。这种高能电子辐射出同步加速器光子，其中的能量将进入较低的能带，高达GeV的伽马射线，可能为极高能量的伽马射线提供证据。然而，这一过程取决于河外空间的磁场强度。因此，仍有更详细的调查有待进行。另一个可能出现的有趣特征是电子能谱中的光谱断裂，这是由于电子与背景辐射的光子碰撞时从汤姆逊散射过渡到康普顿散射。级联过程引起入射伽马射线的相当大的角度扩散，随着高能伽马射线天文学扩展到更高的能量区域和更远的距离，具有宽视场的望远镜预计将变得越来越重要。

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.