ICRF与Gaia-CRF参考架特性分析

Overall Properties of the ICRF and Gaia-CRF
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School of Astronomy & Space Science, Nanjing University, Nanjing 210023; SYRTE, Observatoire de Paris, Paris 75014

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摘要:

天体测量学是天文学中最古老的分支之一, 它是天文学的基础, 对天体物理学和大地测量学的许多方面至关重要.天体测量学的核心任务之一就是建立一个高精度天文参考系, 以便利用这样的惯性参照系来描述天体的位置和运动、研究银河系的运动学特征及对同一天体在不同波段的对应体进行位置认证和比较.当前的天文参考系是参照银河系外天体(主要是活动星系核)的位置来定义的, 称为国际天球参考系(ICRS).在实测上, 与之相对应的基本星表为国际天球参考架(ICRF), 它由甚长基线干涉测量(VLBI)技术在S/X、K和X/Ka波段的观测资料解算而得, 位置精度达到数十微角秒($\mathrm{\mu as}$)水平.另一方面, Gaia卫星的观测也将在光学波段建立类似精度的光学参考架(Gaia-CRF).ICRF与Gaia-CRF的连接成为天体测量领域需要解决的重大问题之一, 这要求对ICRF与Gaia-CRF参考架特性进行细致深入的分析. 首先, 分析了ICRF的整体特性.VLBI星表的内部符合精度估计值在$\mathrm{10\mu as}$和$\mathrm{40\mu as}$之间, 依赖于射电源的观测次数.这一结果一方面验证了ICRF3星表给出的位置噪声水平, 也说明了VLBI技术在天体测量方面的潜力.利用Gaia的河外源位置为参考, 分析了历代ICRF星表的外部符合精度, 指出ICRF3 X/Ka波段参考架存在约$\mathrm{200\mu as}$的系统误差.最后, 发明了一种评估射电源全天分布均匀性的量化指标并改进了ICRF定义源筛选策略, 结果表明相较于ICRF2而言, 这一方法能将ICRF轴指向稳定性提高2至3倍.这些工作很好地解释并补充了国际上ICRF3工作组的相关结果. 其次, 研究Gaia-CRF的参考架性质.使用了相对于依巴谷参考架的全局旋转和结合银河系动力学分析两种方法来评估Gaia-CRF1的惯性水平, 指出Gaia-CRF1可能存在约$\mathrm{0.3mas\cdot yr^{-1}}$的剩余旋转.对于Gaia-CRF2, 研究了其系统精度与河外源样本极限星等的依赖关系, 发现Gaia-CRF2的整体精度几乎不受星等差的影响.这一结果可作为未来ICRF与Gaia-CRF连接源选择的参考. 活动星系核的光学-射电位置差是影响参考架连接精度的重要因素之一.本文首次将光学-射电位置差研究延伸到K和Ka波段, 并研究其与河外源性质参数的相关性, 发现: 光学-射电位置差与星等的相关性是由于星等差而非真实的物理原因造成的, 因此在前人工作中被忽略的暗源也有可能作为ICRF与Gaia-CRF连接源.此外, 本文提出了一种新的参考架连接方法, 即在Gaia-CRF2框架下重新处理VLBI的历史观测资料.先期结果发现使用Gaia-CRF2来替代ICRF3尚不足以显著提高VLBI产品的精度, 但未来在Gaia-CRF参考架精度进一步提高后, 此方法仍值得进一步检验. 本文的研究, 一方面指出了ICRF与Gaia-CRF参考架中可能存在的种种问题, 另一方面也为将来光学与射电参考架连接提供了第一手的参考资料.

Abstract:

The International Celestial Reference System (ICRS) now has realizations at several frequencies, all with an accuracy of several tens of mircoarcsecond ($\mathrm{\mu as}$) level, including the International Celestial Reference Frame (ICRF) based on observations of the very long baseline interferometry (VLBI) at S/X, K, and X/Ka band, and the Gaia celestial reference frame (Gaia-CRF) built by the Gaia mission.The link between the ICRF and Gaia-CRF is a non-trivial task for constructing a multiwavelength reference frame.It requires detailed analyses of the overall properties of both the ICRF and Gaia-CRF for the sake of deepening our understanding of these frames, which is of the main interest of this thesis. The first part is to investigate the internal and external accuracy of ICRF catalogs.The internal accuracy is estimated to range from $\mathrm{10\mu as}$ to $\mathrm{40\mu as}$, depending on the number of observations to individual sources.This analysis provides an independent validation to the documented value ($\mathrm{30\mu as}$) for the noise floor given by the ICRF3 Working Group, as well as indicates the potential accuracy VLBI catalogs could achieve.The external accuracy is studied by comparing all historic ICRF catalogs to the Gaia-CRF2 solution. The large-scale agreement is about $\mathrm{30\mu as}$ and $\mathrm{50\mu as}$ between the ICRF3 S/X band and K band and the Gaia-CRF2 catalogs.The X/Ka band catalog, however, presents deformations of about $\mathrm{200\mu as}$, indicating severe zonal errors in the X/Ka band frame.The possible method of improving the stability of the ICRF axes is also explored, in term of defining source selection.A new selection algorithm considering both source positional stability and uniformness of source distribution is proposed, which is supposed to improve the axis stability by a factor of two compared to the ICRF2. Secondly, different aspects related to the celestial frame in the Gaia Data Release 1 and 2 (DR1 and DR2) are concerned.The non-rotating of the stellar frame of the DR1 is investigated in the light of the global spin of Gaia-CRF1 with respect to the \textscHIPPARCOS frames and using Galactic kinematical analysis.Both methods yield possible residual rotation of around $\mathrm{0.3mas\cdot yr^{-1}}$ in the DR1 frame.As for the DR2, the magnitude-dependent error is estimated to have little influence on the global property of the Gaia-CRF2, suggesting that the overall accuracy of the Gaia-CRF might not degrade like the precision.It underlines a new aspect of the radio-to-optical frame link, which was thought to be taken place only among bright quasars. In preparation for the radio-to-optical frame link, the correlation between the radio-to-optical offsets and source properties are tested statistically to understand its origin.The radio-to-optical offset is found to correlate strongly with the magnitude, however, this correlation would diminish when one consider the position formal error.As a result, large offsets for faint sources are likely resulted from the magnitude-dependent error rather than astrophysical cause.Besides, a method of the radio-to-optical frame link is proposed, whose idea is to introduce the Gaia-CRF in the VLBI data analysis.The preliminary tests suggest no benefit to do so, but it should be re-tested based on the future Gaia data releases with improved accuracy. In conclusion, the agreement among the ICRF3 S/X band frame, K band frame, and Gaia-CRF2 is excellent.However, X/Ka band frame suffers from zonal errors and needs to be improved.More tests on the radio-to-optical frame-tie should be carried out based on the actual Gaia data.

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• 在线发布日期: 2021-12-07
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