The propagation directions of cosmic rays travelling through interstellar space are repeatedly scattered by fluctuating interstellar magnetic fields. The nature of this scattering is a major unsolved problem in astrophysics, one that has resisted solution largely due to a lack of direct observational constraints on the scattering rate. Here we show that very high-energy γ-ray emission from the globular cluster Terzan 5, which has unexpectedly been found to be displaced from the cluster, presents a direct probe of this process. We show that this displacement is naturally explained by cosmic rays accelerated in the bow shock around the cluster, which then propagate a finite distance before scattering processes re-orient enough of them towards Earth to produce a detectable γ-ray signal. The angular distance between the cluster and the signal places tight constraints on the scattering rate, which we show are consistent with a model in which scattering is primarily due to excitation of magnetic waves by the cosmic rays themselves. The analysis method we develop here will make it possible to use sources with similarly displaced non-thermal X-ray and tera-electronvolt γ-ray signals as direct probes of cosmic ray scattering across a range of Galactic environments.

宇宙射线在星际空间中传播的传播方向会被波动的星际磁场反复散射。这种散射的性质是天体物理学中一个未解决的主要问题，该问题一直无法解决，很大程度上是由于缺乏对散射率的直接观测限制。在这里，我们展示了来自球状星团 Terzan 5 的极高能 γ 射线发射，该星团意外地被发现从星团中移出，这为这一过程提供了直接探测。我们表明，这种位移可以自然地用在星团周围的弓激波中加速的宇宙射线来解释，然后宇宙射线传播有限的距离，然后散射过程将足够多的宇宙射线重新定向到地球以产生可检测的γ射线信号。星团和信号之间的角距离对散射率有严格的限制，我们证明这与散射主要是由于宇宙射线本身对磁波的激发引起的模型一致。我们在这里开发的分析方法将使得使用具有类似位移的非热X射线和太电子伏γ射线信号的源作为跨越一系列银河环境的宇宙射线散射的直接探测器成为可能。