The NANOGrav 15 yr Data Set: Targeted Searches for Supermassive Black Hole Binaries

TL;DR

This study reports targeted searches for continuous gravitational waves from 114 active galactic nuclei using 15 years of NANOGrav data, setting new upper limits and illustrating analysis methods.

Contribution

First targeted search incorporating electromagnetic priors, providing improved upper limits and a framework for follow-up of promising candidates.

Findings

Bayesian analysis disfavors CW signals in all targets

Two candidates slightly above noise level but consistent with noise after tests

Updated constraints on a binary in 3C 66B, ruling out part of previous parameter space

Abstract

We present the first targeted searches for continuous gravitational waves (CWs) from 114 active galactic nuclei (AGN) that may host supermassive black hole binaries, using the NANOGrav 15 yr data set. By incorporating electromagnetic priors on sky location, distance, redshift, and CW frequency, our strain and chirp mass upper limits are typically improved by a factor of $\sim 2$ (median 2.2) relative to all-sky limits at the same frequency. Bayesian comparisons against a model including only a Hellings-Downs correlated background disfavors a CW signal for all targets, with a mean Bayes factor of $0.73 \pm 0.32$. Two targets have Bayes factors slightly above unity, but coherence tests, random targeting experiments, and a conservative accounting of the 114-target trials factor all indicate that they are consistent with noise. We use these two candidates as worked examples to illustrate an end-to-end targeted CW search analysis and a suite of follow up tests that future promising candidates would need to pass. We find that the electromagnetic interpretations of both candidates are ambiguous, and we update the constraints on a putative binary in 3C 66B, ruling out part of its previously allowed parameter space. Ultimately, our results demonstrate the current sensitivity of targeted pulsar timing array searches for CWs and define a roadmap for future multimessenger CW detections.