The NANOGrav 15 yr Data Set: Bayesian Limits on Gravitational Waves from Individual Supermassive Black Hole Binaries

Evidence for a low-frequency stochastic gravitational-wave background has recently been reported based on analyses of pulsar timing array data. The most likely source of such a background is a population of supermassive black hole binaries, the loudest of which may be individually detected in these data sets. Here we present the search for individual supermassive black hole binaries in the NANOGrav 15 yr data set. We introduce several new techniques, which enhance the efficiency and modeling accuracy of the analysis. The search uncovered weak evidence for two candidate signals, one with a gravitational-wave frequency of ∼4 nHz, and another at ∼170 nHz. The significance of the low-frequency candidate was greatly diminished when Hellings–Downs correlations were included in the background model. The high-frequency candidate was discounted due to the lack of a plausible host galaxy, the unlikely astrophysical prior odds of finding such a source, and since most of its support comes from a single pulsar with a commensurate binary period. Finding no compelling evidence for signals from individual binary systems, we place upper limits on the strain amplitude of gravitational waves emitted by such systems. At our most sensitive frequency of 6 nHz, we place a sky-averaged 95% upper limit of 8 × 10−15 on the strain amplitude. We also calculate an exclusion volume and a corresponding effective radius, within which we can rule out the presence of black hole binaries emitting at a given frequency.

近期基于对脉冲星计时阵列数据的分析，有报道称发现了低频随机引力波背景的证据。这种背景最有可能的来源是一群超大质量黑洞双星，其中最响亮的可能会在这些数据集中被单独探测到。在此，我们介绍在北美纳赫兹引力波天文台（NANOGrav）15年数据集中对单个超大质量黑洞双星的搜索。我们引入了几种新技术，提高了分析的效率和建模的准确性。搜索发现了两个候选信号的微弱证据，一个引力波频率约为4纳赫兹，另一个约为170纳赫兹。当在背景模型中纳入赫林斯 - 唐斯（Hellings–Downs）相关性时，低频候选信号的显著性大大降低。高频候选信号由于缺乏合理的宿主星系、发现此类源的天体物理学先验概率较低，以及其大部分支持来自一个具有相应双星周期的单脉冲星而被排除。由于没有发现来自单个双星系统信号的有力证据，我们对这类系统发出的引力波应变振幅设定了上限。在我们最敏感的6纳赫兹频率处，我们对应变振幅设定了一个天空平均的95%上限为8×10⁻¹⁵。我们还计算了一个排除体积和相应的有效半径，在此范围内我们可以排除在给定频率下发射的黑洞双星的存在。