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

高能伽马射线与普遍存在的微波和红外波段的宇宙背景辐射相撞，产生电子和正电子对，从而启动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伽马射线天文学的近期前景，研究宇宙中的超高能现象，计算了远距离辐射伽马射线的吸收和再生效应。二次伽马射线的再生导致伽马射线通量在10-100TeV能区积累。然而，当原始伽马射线的能谱与幂指数α=-2一样硬时，通量的增加因子最多约为3，而当α等于2.5时，通量的增加因子小于10%。另一方面，宇宙级联的更显著的效应可能是由级联过程的次要产物电子引起的，电子的能量高于10^{15}eV。这样的高能电子会辐射同步辐射光子，其中的能量会进入与GeV伽马射线一样高的低能带，可能是为了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，仍有更详细的调查有待完成。另一个可能出现的有趣特征是，由于电子与背景辐射的光子的碰撞，电子的能谱从汤姆森散射转变为康普顿散射，级联过程导致入射伽马射线有相当大的角度扩散，随着超高能伽马射线天文学扩展到更高能区和更远的距离，宽视场望远镜有望变得更加重要。

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.