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

Haystack and Owens Valley Radio Observatory (OVRO) observations recently revealed strong 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 P15 GHz=(1739.3±1.2)days. We reject, with p-value = 5.3×10−7, the hypothesis that the variations are due to random fluctuations in the red noise tail of the power spectral density.