The second data release from the European Pulsar Timing Array II. Customised pulsar noise models for spatially correlated gravitational waves

Aims: The nanohertz gravitational wave background (GWB) is expected to be an aggregate signal of an ensemble of gravitational waves emitted predominantly by a large population of coalescing supermassive black hole binaries in the centres of merging galaxies. Pulsar tiNanohertz ming arrays (PTAs), which are ensembles of extremely stable pulsars at approximately kiloparsec distances precisely monitored for decades, are the most precise experiments capable of detecting this background. However, the subtle imprints that the GWB induces on pulsar timing data are obscured by many sources of noise that occur on various timescales. These must be carefully modelled and mitigated to increase the sensitivity to the background signal. Methods: In this paper, we present a novel technique to estimate the optimal number of frequency coefficients for modelling achromatic and chromatic noise, while selecting the preferred set of noise models to use for each pulsar. We also incorporated a new model to fit for scattering variations in the Bayesian pulsar timing package temponest. These customised noise models enable a more robust characterisation of single-pulsar noise. We developed a software package based on tempo2 to create realistic simulations of European Pulsar Timing Array (EPTA) datasets that allowed us to test the efficacy of our noise modelling algorithms. Results: Using these techniques, we present an in-depth analysis of the noise properties of 25 millisecond pulsars (MSPs) that form the second data release (DR2) of the EPTA and investigate the effect of incorporating low-frequency data from the Indian Pulsar Timing Array collaboration for a common sample of ten MSPs. We used two packages, enterprise and temponest, to estimate our noise models and compare them with those reported using EPTA DR1. We find that, while in some pulsars we can successfully disentangle chromatic from achromatic noise owing to the wider frequency coverage in DR2, in others the noise models evolve in a much more complicated way. We also find evidence of long-term scattering variations in PSR J1600-3053. Through our simulations, we identify intrinsic biases in our current noise analysis techniques and discuss their effect on GWB searches. The analysis and results discussed in this article directly help to improve the sensitivity to the GWB signal and they are already being used as part of global PTA efforts.

研究目标：纳赫兹引力波背景（nanohertz gravitational wave background, GWB）被认为是由大量处于并合星系中心的超大质量双黑洞并合产生的引力波集合所形成的累计信号。脉冲星计时阵列（pulsar timing arrays, PTAs）由数十年来被精准监测的、距离约千秒差距的极端稳定脉冲星群组成，是目前最精准的可探测该背景的实验手段。然而，引力波背景在脉冲星计时数据中留下的细微印记，会被多种不同时标下存在的噪声源所掩盖。因此，必须对这些噪声进行精细建模并加以抑制，以提升对背景信号的探测灵敏度。
研究方法：本文提出一种全新的技术，可在为每颗脉冲星优选噪声模型集合的同时，估计用于建模消色差噪声与色差噪声的最优频率系数数量。此外，我们在贝叶斯脉冲星计时软件包temponest中加入了用于拟合散射变化的新模型。这些定制化噪声模型可实现对单颗脉冲星噪声更稳健的表征。我们基于tempo2开发了一款软件包，用于生成欧洲脉冲星计时阵列（European Pulsar Timing Array, EPTA）数据集的逼真仿真结果，以此检验我们的噪声建模算法的有效性。
研究结果：借助上述技术，我们对构成欧洲脉冲星计时阵列第二版数据发布（second data release, DR2）的25颗毫秒脉冲星（millisecond pulsars, MSPs）的噪声特性开展了深入分析，并针对10颗共源脉冲星样本，探究了加入印度脉冲星计时阵列合作组提供的低频数据所带来的影响。我们采用enterprise与temponest两款软件包估算噪声模型，并将其与欧洲脉冲星计时阵列第一版数据发布（EPTA DR1）中的结果进行对比。研究发现，得益于第二版数据发布更宽的频率覆盖范围，部分脉冲星中的色差噪声与消色差噪声可被成功分离，但另有部分脉冲星的噪声模型演化过程则更为复杂。我们还在PSR J1600-3053中发现了长期散射变化的证据。通过仿真实验，我们识别出现有噪声分析技术中存在的内在偏差，并讨论了其对引力波背景搜寻工作的影响。本文所讨论的分析方法与研究结果可直接提升引力波背景信号的探测灵敏度，且已被纳入全球脉冲星计时阵列合作项目的工作之中。