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Blandford, R. Maiolino, S. Kiehlmann, S. K. N{\\ae}ss, S. O'Neill, S. R. Kulkarni, T. A. Prince, T. Hovatta, T. J. Pearson, T. J. W. Lazio, V. Pavlidou, V. Ravi, W. Max-Moerbeck, Y. Ding","submitted_at":"2024-07-12T19:05:24Z","abstract_excerpt":"Haystack and Owens Valley Radio Observatory (OVRO) observations recently revealed strong, intermittent, sinusoidal total flux-density variations that maintained coherence between 1975 and 2021 in the blazar PKS 2131$-$021 ($z=1.283$). This was interpreted as possible evidence of a supermassive black hole binary (SMBHB). Extended observations through 2023 show coherence over 47.9 years, with an observed period $P_\\textrm{15 GHz}=(1739.8 \\pm 17.4)$ days}. 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