In 2017 April, the Event Horizon Telescope (EHT) observed the near-horizon region around the supermassive black hole at the core of the M87 galaxy. These 1.3 mm wavelength observations revealed a compact asymmetric ring-like source morphology. This structure originates from synchrotron emission produced by relativistic plasma located in the immediate vicinity of the black hole. Here we present the corresponding linear-polarimetric EHT images of the center of M87. We find that only a part of the ring is significantly polarized. The resolved fractional linear polarization has a maximum located in the southwest part of the ring, where it rises to the level of ∼15%. The polarization position angles are arranged in a nearly azimuthal pattern. We perform quantitative measurements of relevant polarimetric properties of the compact emission and find evidence for the temporal evolution of the polarized source structure over one week of EHT observations. The details of the polarimetric data reduction and calibration methodology are provided. We carry out the data analysis using multiple independent imaging and modeling techniques, each of which is validated against a suite of synthetic data sets. The gross polarimetric structure and its apparent evolution with time are insensitive to the method used to reconstruct the image. These polarimetric images carry information about the structure of the magnetic fields responsible for the synchrotron emission. Their physical interpretation is discussed in an accompanying publication.

2017 年 4 月，事件视界望远镜 (EHT) 观测到了 M87 星系核心超大质量黑洞周围的近地平线区域。这些 1.3 毫米波长的观察揭示了一个紧凑的不对称环状源形态。这种结构源于位于黑洞附近的相对论等离子体产生的同步加速器发射。在这里，我们展示了 M87 中心的相应线性偏振 EHT 图像。我们发现只有环的一部分被显着极化。分辨的线性极化在环的西南部分有一个最大值，在那里它上升到 15% 的水平。偏振位置角以接近方位角的方式排列。我们对紧凑发射的相关偏振特性进行了定量测量，并在一周的 EHT 观测中找到了偏振源结构时间演变的证据。提供了极化数据缩减和校准方法的详细信息。我们使用多种独立的成像和建模技术进行数据分析，每种技术都针对一组合成数据集进行验证。粗略的极化结构及其随时间的明显演变对用于重建图像的方法不敏感。这些极化图像携带有关负责同步加速器发射的磁场结构的信息。它们的物理解释在随附的出版物中进行了讨论。