| Description: |
Supermassive black hole binary systems (SMBHBs) are thought to emit the recently discovered nHz gravitational wave background; however, not a single individual nHz source has been confirmed to date. Long-term radio-monitoring at the Owens Valley Radio Observatory has revealed two potential SMBHB candidates: blazars PKS 2131-021 and PKS J0805-0111. These sources show periodic flux density variations across the electromagnetic spectrum, signaling the presence of a good clock. To explain the emission, we propose a generalizable jet model, where a mildly relativistic wind creates an outward-moving helical channel, along which the ultrarelativistic jet propagates. The observed flux variation from the jet is mostly due to aberration. The emission at a lower frequency arises at a larger radius, and its variation is consequently delayed, as observed. Our model reproduces the main observable features of both sources and can be applied to other sources as they are discovered. We make predictions for radio polarization, direct imaging, and emission line variation, which can be tested with forthcoming observations. Our results motivate future numerical simulations of jetted SMBHB systems and have implications for the fueling, structure, and evolution of blazar jets. ; © 2026. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence . Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. ; The authors thank Alisa Galishnikova, Martijn Oei, Timothy J. Pearson, Viraj Manwadkar, Jack Dinsmore, Roger W. Romani, Emanuele Sobacchi, Alexander Philippov, Lorenzo Sironi, and Paulo Coppi for useful discussions. The authors also thank the anonymous referee for useful feedback, which improved this manuscript. This work was supported in part by a grant from the Simons Foundation (00001470, R.B., A.G.S., N.G.). A.G.S. acknowledges the support of ... |