The Universe is largely transparent to $$\gamma $$γ-rays in the GeV energy range, making these high-energy photons valuable for exploring energetic processes in the cosmos. After 7 years of operation, the Fermi Gamma-ray Space Telescope has produced a wealth of information about the high-energy sky. This review focuses on extragalactic $$\gamma $$γ-ray sources: what has been learned about the sources themselves and about how they can be used as cosmological probes. Active galactic nuclei (blazars, radio galaxies, Seyfert galaxies) and star-forming galaxies populate the extragalactic high-energy sky. Fermi observations have demonstrated that these powerful non-thermal sources display substantial diversity in energy spectra and temporal behavior. Coupled with contemporaneous multifrequency observations, the Fermi results are enabling detailed, time-dependent modeling of the energetic particle acceleration and interaction processes that produce the $$\gamma $$γ-rays, as well as providing indirect measurements of the extragalactic background light and intergalactic magnetic fields. Population studies of the $$\gamma $$γ-ray source classes compared to the extragalactic $$\gamma $$γ-ray background place constraints on some models of dark matter. Ongoing searches for the nature of the large number of $$\gamma $$γ-ray sources without obvious counterparts at other wavelengths remain an important challenge.

中文翻译：

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费米时代的河外伽马射线天空

宇宙在很大程度上对 GeV 能量范围内的 $$\gamma $$γ 射线是透明的，这使得这些高能光子对于探索宇宙中的能量过程很有价值。费米伽马射线太空望远镜经过 7 年的运行，已经产生了大量关于高能天空的信息。这篇综述的重点是河外 $$\gamma $$γ 射线源：关于这些源本身以及如何将它们用作宇宙学探测器的知识。活跃的星系核（耀变体、射电星系、赛弗特星系）和恒星形成星系遍布河外高能天空。费米观测表明，这些强大的非热源在能谱和时间行为方面表现出巨大的多样性。再加上同时进行的多频观测，费米结果能够提供详细的、对产生 $$\gamma $$γ 射线的高能粒子加速和相互作用过程的时间相关建模，以及提供对河外背景光和星际磁场的间接测量。与河外 $$\gamma $$γ 射线背景相比，$$\gamma $$γ 射线源类别的种群研究对某些暗物质模型施加了限制。对大量 $$\gamma $$γ 射线源在其他波长没有明显对应物的性质的持续搜索仍然是一个重要的挑战。与河外 $$\gamma $$γ 射线背景相比，$$\gamma $$γ 射线源类别的种群研究对某些暗物质模型施加了限制。对大量 $$\gamma $$γ 射线源在其他波长没有明显对应物的性质的持续搜索仍然是一个重要的挑战。与河外 $$\gamma $$γ 射线背景相比，$$\gamma $$γ 射线源类别的种群研究对某些暗物质模型施加了限制。对大量 $$\gamma $$γ 射线源在其他波长没有明显对应物的性质的持续搜索仍然是一个重要的挑战。