发表机构
GFZ Helmholtz Centre for Geosciences; Max Planck Institute for Radio Astronomy（亥姆霍兹波茨坦地学研究中心; 马克斯·普朗克射电天文学研究所）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
本研究将Gaia光学质心配准到VLBI图像，精度约0.1毫角秒，测得B 1038+528/529的8.4 GHz核位移分别为169和293微角秒，验证了核位移由不透明度驱动，方法适用于有Gaia探测和VLBI天体测量的AGNs。
AI 中文摘要
由于活动星系核（AGNs）的射电结构复杂且随时间变化，绝对天体测量位置通常无法明确地与VLBI图像中的特定特征配准。我们旨在将Gaia光学质心配准到VLBI图像上，精度约为0.1毫角秒，并确定类星体对B 1038+528和B 1038+529中光学发射相对于射电核的位置，在该系统中首次发现了不透明度驱动的核位移。我们结合了1981年至1995年间四个历元的相位参考VLBI测得的该对类星体的相对位置、它们的Gaia光学位置以及几乎正交的射电喷流方向。假设每个Gaia-核位移都沿局部喷流轴方向，我们在不使用绝对VLBI位置的情况下，将Gaia质心相对于射电核配准，精度约为0.1毫角秒。Gaia质心位于8.4 GHz射电核的上游，在B 1038+529中为293 ± 93微角秒，在B 1038+528中为169 ± 96微角秒。后者与四十年前对该类星体测得的2.3–8.4 GHz位移所隐含的260微角秒8.4 GHz核位移一致。B 1038+528的VLBI绝对天体测量位置位于核下游约0.6毫角秒处，其大地测量位置时间序列沿喷流漂移超过1毫角秒，表明该对类星体的0.8–1.6毫角秒Gaia-VLBI偏移主要由源结构主导，而非核位移。Gaia位置可以以天体测量不确定性所决定的精度配准到VLBI图像上。如果Gaia质心标志着喷流底部，则测得的间隔即为绝对的8.4 GHz核位移，对应于距喷流底部约1.2秒差距和约2.5秒差距的投影距离；光学喷流发射的贡献将使这些值成为下限。该方法适用于具有Gaia探测和相位参考VLBI天体测量的AGNs。
英文摘要
Because of the complex and time-variable radio structure of active galactic nuclei (AGNs), absolute astrometric positions cannot generally be registered unambiguously with specific features in VLBI images. We aim to register Gaia optical centroids on VLBI images with about 0.1 mas accuracy and to locate the optical emission relative to the radio core in the quasar pair B 1038+528 and B 1038+529, in which the opacity-driven core shift was first discovered. We combine the relative position of the pair from phase-referenced VLBI at four epochs between 1981 and 1995, their Gaia optical positions, and the nearly orthogonal radio-jet directions. Assuming that each Gaia--core displacement lies along the local jet axis, we register the Gaia centroids relative to the radio cores with about 0.1 mas accuracy without using absolute VLBI positions. The Gaia centroids lie upstream of the 8.4-GHz radio cores, by 293 {\pm} 93 μas in B 1038+529 and 169 {\pm} 96 μas in B 1038+528. The latter agrees with the 260 μas 8.4-GHz core shift implied by the 2.3--8.4 GHz shift measured for this quasar four decades ago. The absolute astrometric position of B 1038+528 with VLBI lies about 0.6 mas downstream of the core, and its geodetic position time series drifts along the jet by more than 1 mas, showing that the 0.8--1.6 mas Gaia--VLBI offsets of the pair are dominated by source structure rather than by core shift. Gaia positions can be registered on VLBI images with an accuracy set by the astrometry uncertainties. If the Gaia centroids mark the jet base, the measured separations are absolute 8.4-GHz core shifts, corresponding to projected distances of about 1.2 pc and about 2.5 pc from the jet base; a contribution from an optical jet emission would make them lower limits. The method is applicable to AGNs with Gaia detection and phase-referenced VLBI astrometry.
Comments7 pages, 4 figures, submitted to A&A
