2026ApJ...998L..42C

A subparsec supermassive black hole binary (SMBHB) at the center of the galaxy 3C 66B is a promising candidate for continuous gravitational-wave searches with pulsar timing arrays (PTAs). In this work, we search for such a signal in the third data release of the Parkes Pulsar Timing Array. Matching our priors to estimates of binary parameters from electromagnetic (EM) observations, we find a log Bayes factor $\mathrm{ln}{ \mathcal B }=-0.0027(7)$, highlighting that the source can be neither confirmed nor ruled out. We place upper limits at 95% credibility on the chirp mass ${ \mathcal M }\lt 6.90\times 1{0}^{8}\,{M}_{\odot }$, and on the characteristic strain amplitude ${{\rm{log}}}_{10}({h}_{0})\lt -14.44$. This partially rules out the parameter space suggested by EM observations of 3C 66B. We also independently reproduce the calculation of the chirp mass with 3 mm flux monitor data of the unresolved core of 3C 66B. Based on this, we outline a new methodology for constructing a joint likelihood of EM and gravitational-wave data from SMBHBs. Finally, we suggest that targeted searches may allow firmly established SMBHB candidates to be treated as standard sirens, for complementary constraints on the Universe's expansion rate.