对毫秒脉冲星计时中的红噪声的解析研究

The pulsar timing is an important technique in researches of detecting magnetic field of galactic system and gravatitional waves, pulsar glitches, radiation properties of pulsars, interstellar scintillations and scattering of pulsars and so on. The accuracy of timing residuals arrives 100 ns based on timing observation data with 5 year span. However, obvious low-frequency noises,or red noises, exist in the timing residuals, which is the essential issue for related researches and applications. At present the researchers always fit the red noises by methematic methods before parameter reduction, which is not related to the phycial background of red noises at all. Moreover, the important systematic model errors are deleted during such fitting. From generating mechanism of red noises, we will describe the models of the red noises and the pulsar data sets will be processed in a manner that will simutaneously identify the global facters between pulsars including irregularites in the time standard, planet mass errors in the Solar system ephemeris, gravitational waves and so on. We will correct the other random facters between pulsars such ionospheric errors by the means of GPS dual-band observations data. The remaining random facters between pulsars will be fitted by methematic method. And such procedures will effectively reduce the effects of red noises. Besides, we will develop related software and put it to present TEMPO2 software package as plug-in components. At the same time, We will validate the improvement accuracy of our methods on red noises by reduction of the public timing data from international network by Parkes radio antenna and other antennas, and also the new millisecond-pulsar data from Shanghai Tianma 65m radio telescope.

脉冲星计时是开展相关物理研究的重要方法。尽管实测毫秒脉冲星计时残差可达几十纳秒，而其中存在的显著的红噪声，已成为相关研究及应用中亟待解决的难题。国际上普遍在参数解算前采用数学拟合方法白化红噪声，这只是纯数学的消除，无物理意义，且红噪声整体性因素的相互影响也在白化中被消除。本项目将从红噪声的产生机制出发，首先模型上作合理描述，对时间尺度和转换误差、太阳系历表的行星质量误差及引力波背景影响等星间整体性因素作联合求解；对能用实测方法消除的星间随机性因素先行扣除，如将GPS观测得到的电离层修正用于星际介质误差的改进；对于无法扣除仅与脉冲星内禀特征有关因素做白化处理，这将有效降低或削弱红噪声的影响。此外编制相关软件作为插件加入至现有计时软件系统中。基于国际已有计时数据和上海65m望远镜新增数据验证该方法对红噪声的改进精度，这在引力波探测、脉冲星时间系统建立、太阳系行星质量改进等领域具有重要意义。

国际脉冲星计时阵IPTA的观测数据是目前计时精度最高的，有些毫秒脉冲星的计时残差可达几十纳秒，但是很多毫秒脉冲星都存在着不同水平的红噪声。本项目首先对IPTA已经公布的49颗毫秒脉冲星的计时数据分别进行了噪声模型分析，包括重新定标每个脉冲星的白噪声水平，采用split-Cholesky方法分析红噪声，包括先后对与频率相关的DM噪声以及频率无关的红噪声进行各自建模，由此得到每个脉冲星的模型参数。基于这些噪声研究，本项目主要开展了建立脉冲星时间系统和太阳系动力学参考架与ICRF连接的工作。.尽管TAI的精度和稳定度在逐步提高，但是长期稳定性难以维持，比较不同原子钟的差异，难以给出自身的精度和稳定度比较，而脉冲星具有高精度的可预测性，长期稳定度好，因此基于脉冲星计时观测可以建立起具有长期稳定性的独立的时间系统。项目中我们利用IPTA的49颗毫秒脉冲星的多波段计时数据，首先得到各个脉冲星的噪声分析，基于频率分析方法，利用整体法解算得到最新的脉冲星时间系统，相比于之前发表的脉冲星时间系统，精度有大幅提高。并且基于贝叶斯方法，进行了分析比对，得到了一致的结果，也再次证明了TT（BIPM16）是当时精度最好的时间系统。随着IPTA计时精度的逐步提高，我们期待能够检测到TT（BIPM）时间系统的误差。.此外，基于共同有计时观测和VLBI相位参考观测的多颗毫秒脉冲星的数据，我们发展了连接行星历表和ICRF的新的方法，将参考架连接参数和地球日心距误差参数加入至脉冲星计时模型中，同时解算出参考架连接参数和地球日心距参数，基于具有同时计时和VLBI观测的4颗毫秒脉冲星的数据得到了DE200和ICRF的连接参数，该结果与间接连接方法得到的结果一致。