The NANOGrav 15-Year Data Set: Detector Characterization and Noise Budget

Pulsar timing arrays (PTAs) are galactic-scale gravitational wave detectors. Each individual arm, composed of a millisecond pulsar, a radio telescope, and a kiloparsecs-long path, differs in its properties but, in aggregate, can be used to extract low-frequency gravitational wave (GW) signals. We present a noise and sensitivity analysis to accompany the NANOGrav 15-year data release and associated papers, along with an in-depth introduction to PTA noise models. As a first step in our analysis, we characterize each individual pulsar data set with three types of white noise parameters and two RN parameters. These parameters, along with the timing model and, particularly, a piecewise-constant model for the time-variable dispersion measure, determine the sensitivity curve over the low-frequency GW band we are searching. We tabulate information for all of the pulsars in this data release and present some representative sensitivity curves. We then combine the individual pulsar sensitivities using a signal-to-noise-ratio statistic to calculate the global sensitivity of the PTA to a stochastic background of GWs with a minimum noise characteristic strain of 7×10^{−15} at 5 nHz. A power law-integrated analysis shows rough agreement with the amplitudes recovered in NANOGrav’s 15-year GW background analysis. While our phenomenological noise model does not model all known physical effects explicitly, it provides an accurate characterization of the noise in the data while preserving sensitivity to multiple classes of GW signals.

脉冲星计时阵列（Pulsar timing arrays, PTAs）是银河尺度的引力波探测器。每个独立探测臂由一颗毫秒脉冲星、一台射电望远镜以及一段千秒差距级的传播路径构成，各臂特性存在差异，但整体可用于提取低频引力波（gravitational wave, GW）信号。本文针对北美纳赫兹引力波天文台（NANOGrav）15年数据发布及相关配套论文开展了噪声与灵敏度分析，并对PTA噪声模型进行了深入介绍。作为本次分析的第一步，我们利用三类白噪声参数与两类红噪声（RN）参数对每颗独立脉冲星的数据集进行特性表征。这些参数结合计时模型，尤其是针对时变色散量的分段常数模型，决定了我们所搜寻的低频引力波频段内的灵敏度曲线。我们为本次数据发布中的所有脉冲星整理了相关信息，并展示了若干代表性的灵敏度曲线。随后我们采用信噪比统计量合并各脉冲星的灵敏度，以此计算PTA对随机引力波背景的全局灵敏度，该背景在5纳赫兹处的最小噪声特征应变为7×10⁻¹⁵。幂律积分分析结果与NANOGrav 15年引力波背景分析中恢复的振幅大致相符。尽管我们的唯象噪声模型并未显式建模所有已知物理效应，但它能够准确表征数据中的噪声特性，同时保留对多类引力波信号的探测灵敏度。