标题： 银河系中心磁星PSR J1745--2900的长期天文测量监测 显示英文标题

标题： Long-Term Astrometric Monitoring of the Galactic Center Magnetar PSR J1745--2900

摘要： 我们展示了对银河系中心磁星PSR J1745-2900的新的甚长基线阵列（VLBA）天体测量观测结果。结合之前发表的跨越477天的10个历元的数据，完整数据集包括25个历元和41个独立测量，覆盖了1984天的时间。这些数据限制了自行精度达到$\lesssim 2\%$，并在两个天球坐标上设定了磁星加速度绝对值的上限为$\lesssim (0.4, 0.2)\, {\rm mas\,y^{-2}}$，与围绕Sgr A*轨道预期的最大值$\sim 0.03\,{\rm mas\,y^{-2}}$一致。未来若PSR J1745-2900重新变亮，则有望检测到由Sgr A*引起的加速度。我们在拟合标准参数后考虑了几种潜在的系统性变化来源，包括折射漂移、Sgr A*结构的变化以及未见伴星的存在。尽管可以为天体测量数据拟合一颗恒星伴星模型，但脉冲周期测量与该模型不符。在这次观测期间未检测到磁星表观图像大小的变化，表明视线散射性质在此期间没有变化。我们还表明Sgr A*平均核心位移的上限与紧凑喷流或对称吸积流的预期一致。 显示英文摘要

摘要： We present new astrometric observations of the Galactic Center magnetar, PSR J1745-2900, with the Very Long Baseline Array (VLBA). Combined with previously published measurements in 10 epochs that spanned 477 days, the complete data set consists of 25 epochs and 41 independent measurements that span 1984 days. These data constrain the proper motion to an accuracy of $\lesssim 2\%$ and set an upper limit on the absolute value of the magnetar's acceleration of $\lesssim (0.4, 0.2)\, {\rm mas\,y^{-2}}$ in the two celestial coordinates, consistent with the maximum value of $\sim 0.03\,{\rm mas\,y^{-2}}$ expected for an orbit around Sgr A*. Future measurements have the potential to detect the acceleration of PSR J1745-2900 due to Sgr A* should PSR J1745-2900 re-brighten. We consider several potential sources of systematic variations in the astrometric residuals after fitting for standard parameters, including refractive wander, changes in the structure of Sgr A*, and the presence of an unseen binary companion. While a stellar companion model can be fit to the astrometric data, pulse period measurements are inconsistent with that model. No changes in the apparent image size of the magnetar were detected over the duration of these observations, indicating a lack of change in the properties of the line-of-sight scattering during this period. We also show that the upper limit to the mean core shift of Sgr A* is consistent with expectations for a compact jet or symmetric accretion flow.