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 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.

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2024
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