2025ApJ...992...60V

We present new constraints on the physical properties of close binary supermassive black holes (SMBHs) in the active galactic nuclei 3C 273, 3C 454.3, S0528+134, and AO 0235+164 based on long-term, multifrequency radio monitoring. By applying a combined time-series analysis using Fourier and wavelet techniques, we identify quasiperiodic variations indicative of orbital motion in compact binary SMBH systems. The derived parameters point to highly compact configurations, with total system masses reaching up to ∼2.4 · 10¹⁰M_⊙ and near-equal mass ratios. The inferred orbital separations and periods suggest that these binaries are in a late inspiral stage, emitting gravitational waves in the nanohertz range. Estimated strain amplitudes indicate that such sources may be detectable by current and upcoming pulsar timing arrays, including IPTA, NANOGrav, and EPTA. These results support the hypothesis that a subset of bright blazars host tightly bound SMBH pairs and represent a promising population of low-frequency gravitational-wave sources.