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

Abstract

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.

Metastable cosmic strings

“…In order to distinguish metastable cosmic strings from other interpretations of the SGWB signal at PTA frequencies, a more precise determination of the spectral tilt will be important. Moreover, like most other cosmological signals, the SGWB from metastable cosmic strings is largely isotropic, as opposed to the significant anisotropies, and the possible presence of resolvable sources, which are expected for a GW signal from supermassive black-hole binaries [92][93][94]. Future pulsar observations and combinations of existing PTA data sets will shed light on these questions in the near future.…”

How to Detect an Astrophysical Nanohertz Gravitational Wave Background

“…We can also see that the recovery of some parameters shows nonnegligible bias. This run is well converged, and QuickCW has been shown to produce unbiased parameter estimates (Agazie et al 2023f) in CURN+CW simulations. Thus, we suspect that this bias is due to some model mis-specification, e.g., because the background is modeled with a power-law spectrum, or because we assume that only one significant binary is present.…”

“…It is important to note that while the NANOGrav 15 yr analysis recovers a γ GWB value lower than 13/3, this does not significantly change with the inclusion of an individual binary model (Agazie et al 2023f), and the γ GWB recovery is instead…”

“…We selected this realization randomly under the constraints that it has a CW S/N > 5 and a monopole S/N > 1.5. The former ensured that there is a detectable individual binary in the data, while the latter was motivated by the subdominant monopolar signature found around 4 nHz in the NANOGrav 15 yr data set (Agazie et al 2023a), which was also identified by the individual binary search carried out using that data set (Agazie et al 2023f). The selected realization has a monopole S/N of 1.8, a dipole S/N of −1.33, an HD S/N of 2.29, an HD versus CURN BF of 77, a = --A log 14.4 10 0.1yr 1 , a CW S/N of 6.2, and a CW frequency of 9 nHz.…”

Pulsar Timing Array signature from oscillating metric perturbations due to ultra-light axion

“…However, a plethora of alternative explanations are also possible [30][31][32]42]. Although the current dataset shows no strong evidence for anisotropies [43] nor gravitational waves from individual SMBH binaries [44], combining observations in IPTA and accumulating more data would allow an in-depth study of nano-hertz band gravitational waves.…”