Ultrahigh-energy cosmic rays (UHECRs) are atomic nuclei from space with vastly higher energies than any other particles ever observed. Their origin and chemical composition remain a mystery. As we show here, the large and intermediate angular scale anisotropies observed by the Pierre Auger Observatory are a powerful tool for understanding the origin of UHECRs. Without specifying any particular production mechanism but only postulating that the source distribution follows the matter distribution of the local universe, a good accounting of the magnitude, direction, and energy dependence of the dipole anisotropy at energies above 8 10¹⁸ eV is obtained after taking into account the impact of energy losses during propagation (the “GZK horizon”), diffusion in the extragalactic magnetic field, and deflections in the Galactic magnetic field (GMF). This is a major step toward the long-standing hope of using UHECR anisotropies to constrain UHECR composition and magnetic fields. The observed dipole anisotropy is incompatible with a pure proton composition in this scenario. With a more accurate treatment of energy losses, it should be possible to further constrain the cosmic-ray composition and properties of the extragalactic magnetic field, self-consistently improve the GMF model, and potentially expose individual UHECR sources.

超高能宇宙射线 (UHECR) 是来自太空的原子核，其能量比以往任何其他粒子都高得多。它们的起源和化学成分仍然是个谜。正如我们在此展示的，皮埃尔奥格天文台观测到的大中型角尺度各向异性是了解 UHECR 起源的有力工具。没有具体说明任何特定的产生机制，只是假设源分布遵循局部宇宙的物质分布，很好地说明了能量高于 8 10 ^{18 时}偶极各向异性的大小、方向和能量依赖性eV 是在考虑了传播过程中的能量损失（“GZK 视界”）、河外磁场中的扩散和银河磁场 (GMF) 中的偏转的影响后获得的。这是朝着使用 UHECR 各向异性限制 UHECR 成分和磁场的长期希望迈出的重要一步。在这种情况下，观察到的偶极子各向异性与纯质子组成不相容。通过更准确地处理能量损失，应该有可能进一步约束银河系外磁场的宇宙射线成分和特性，自洽地改进 GMF 模型，并可能暴露单个 UHECR 源。