Fast radio bursts (FRBs) are bright, coherent, short-duration radio transients of as-yet unknown extragalactic origin. FRBs exhibit a variety of spectral, temporal and polarimetric properties that can unveil clues into their emission physics and propagation effects in the local medium. Here, we present the high-time-resolution (down to 1 μs) polarimetric properties of four 1.7 GHz bursts from the repeating FRB 20180916B, which were detected in voltage data during observations with the European Very Long Baseline Interferometry Network. We observe a range of emission timescales that spans three orders of magnitude, with the shortest component width reaching 3–4 μs (below which we are limited by scattering). We demonstrate that all four bursts are highly linearly polarized (≳80%), show no evidence of significant circular polarization (≲15%), and exhibit a constant polarization position angle (PPA) during and between bursts. On short timescales (≲100 μs), however, there appear to be subtle PPA variations (of a few degrees) across the burst profiles. These observational results are most naturally explained in an FRB model in which the emission is magnetospheric in origin, in contrast to models in which the emission originates at larger distances in a relativistic shock.

快速射电暴 (FRB) 是明亮、连贯、持续时间短的射电瞬变，其起源至今未知。FRB 表现出多种光谱、时间和偏振特性，可以揭示它们在局部介质中的发射物理和传播效应的线索。在这里，我们展示了来自重复 FRB 20180916B 的四个 1.7 GHz 突发的高分辨率（低至 1 μs）偏振特性，这些特性是在欧洲超长基线干涉测量网络观测期间在电压数据中检测到的。我们观察到一系列跨越三个数量级的发射时间尺度，最短的组件宽度达到 3-4 μs（低于此范围，我们受到散射的限制）。我们证明所有四个突发都是高度线性极化的（≳80%），没有显示出明显的圆极化（≲ 15%），并且在爆发期间和爆发之间表现出恒定的极化位置角（PPA）。然而，在较短的时间尺度上（≲ 100 μs），整个爆发剖面似乎存在细微的 PPA 变化（几度）。这些观测结果最自然地在 FRB 模型中得到解释，在该模型中，发射起源于磁层，与在相对论激波中发射起源于更远距离的模型形成对比。