Abstract Galaxy clusters host the largest particle accelerators in the Universe: Shock waves in the intracluster medium (ICM), a hot and ionised plasma, that accelerate particles to high energies. Radio observations pick up synchrotron emission in the ICM, proving the existence of accelerated cosmic-ray electrons. However, a sign of cosmic-ray protons, in form of γ -rays. remains undetected. This is know as the missing γ -ray problem and it directly challenges the shock acceleration mechanism at work in the ICM. Over the last decade, theoretical and numerical studies focused on improving our knowledge on the microphysics that govern the shock acceleration process in the ICM. These new models are able to predict a γ -ray signal, produced by shock accelerated cosmic-ray protons, below the detection limits set modern γ -ray observatories. In this review, we summarise the latest advances in solving the missing γ -ray problem.

中文翻译：

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论簇内介质中宇宙射线质子激波加速的挑战

摘要 星系团拥有宇宙中最大的粒子加速器：团内介质 (ICM) 中的冲击波，一种热的电离等离子体，可将粒子加速到高能量。无线电观测在 ICM 中接收到同步加速器发射，证明加速宇宙射线电子的存在。然而，宇宙射线质子的标志，以γ射线的形式存在。仍然未被发现。这被称为缺失 γ 射线问题，它直接挑战了 ICM 中工作的冲击加速机制。在过去的十年中，理论和数值研究的重点是提高我们对控制 ICM 中冲击加速过程的微观物理学的认识。这些新模型能够预测由冲击加速的宇宙射线质子产生的 γ 射线信号，其低于现代 γ 射线天文台设定的检测极限。