PKS 2131-021 -- Discovery of Strong Coherent Sinusoidal Variations from Radio to Optical Frequencies: Compelling Evidence for a Blazar Supermassive Black Hole Binary

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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}.
• We reject, with $p$-value = $2.09 \times 10^{-7}$, the hypothesis that the variations are due to random fluctuations in the red noise tail of the power spectral density.