A gravitational wave event, S190510g, which was classified as a binary-neutron-star coalescence at the time of preliminary alert, was detected by LIGO/Virgo collaboration on 2019 May 10. At 1.7 hours after the issue of its preliminary alert, we started a target-of-opportunity imaging observation in the Y band to search for its optical counterpart using the Hyper Suprime-Cam (HSC) on the Subaru Telescope. The observation covers a 118.8 deg2 sky area corresponding to $11.6\%$ confidence in the localization skymap released in the preliminary alert and $1.2\%$ in the updated skymap. We divided the observed area into two fields based on the availability of HSC reference images. For the fields with the HSC reference images, we applied an image subtraction technique; for the fields without the HSC reference images, we sought individual HSC images by matching a catalog of observed objects with the PS1 catalog. The search depth is 22.28 mag in the former method and the limit of search depth is 21.3 mag in the latter method. Subsequently, we performed visual inspection and obtained 83 candidates using the former method and 50 candidates using the latter method. Since we only have the one-day photometric data, we evaluated the probability of candidates being located inside the 3D skymap by estimating their distances with photometry of associated extended objects. We found three candidates are likely located inside the 3D skymap and concluded they could be a counterpart of S190510g, while most of the 133 candidates were likely to be supernovae because the number density of candidates was consistent with the expected number of supernova detections. By comparing our observational depth with a light-curve model of such a kilonova reproducing AT2017gfo, we show that early deep observations with the Subaru/HSC can capture the rising phase of the blue component of a kilonova at the estimated distance of S190510g (∼230 Mpc).

中文翻译：

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使用 Subaru/Hyper Suprime-Cam 对 GW 事件 S190510g 进行光学跟踪观察

LIGO/Virgo 合作于 2019 年 5 月 10 日探测到一个引力波事件 S190510g，在初步警报时被归类为双中子星合并。在其初步警报发布 1.7 小时后，我们开始使用斯巴鲁望远镜上的 Hyper Suprime-Cam (HSC) 在 Y 波段进行机会目标成像观察，以寻找其光学对应物。观测覆盖了 118.8 度的天空区域，对应于初步警报中发布的定位天空图的 $11.6\%$ 置信度和更新后的天空图中的 $1.2\%$。我们根据 HSC 参考图像的可用性将观察区域分为两个区域。对于具有 HSC 参考图像的字段，我们应用了图像减法技术；对于没有 HSC 参考图像的字段，我们通过将观察到的物体目录与 PS1 目录进行匹配来寻找单个 HSC 图像。前一种方法的搜索深度为22.28 mag，后一种方法的搜索深度限制为21.3 mag。随后，我们进行了目视检查，使用前一种方法获得了 83 名候选者，使用后一种方法获得了 50 名候选者。由于我们只有一天的光度数据，我们通过使用相关扩展对象的光度测量来估计候选者位于 3D 天空图内的概率。我们发现三个候选者可能位于 3D 天空图中，并得出结论它们可能是 S190510g 的对应物，而 133 个候选者中的大多数可能是超新星，因为候选者的数量密度与预期的超新星探测数量一致。