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

TL;DR

This paper reports the discovery of persistent, coherent sinusoidal flux variations across radio to optical frequencies in the blazar PKS 2131-021, providing strong evidence for a supermassive black hole binary system.

Contribution

It presents the first multi-frequency, long-term evidence of sinusoidal flux variations consistent with a SMBHB in a blazar, with detailed analysis ruling out random noise.

Findings

Coherent sinusoidal variations observed over 47.9 years

Period of approximately 1740 days across frequencies

Hints of periodicity at gamma-ray energies

Abstract

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. There is clearly a physical phenomenon in PKS 2131$-$021 producing coherent sinusoidal flux density variations. We find the coherent sinusoidal intensity variations extend from below 2.7 GHz to optical frequencies, from which we derive an observed period $P_\textrm{optical}=(1764 \pm 36)$ days. Across this broad frequency range there is a smoothly-varying monotonic phase shift in the sinusoidal variations with frequency. Hints of periodic variations are also observed at $\gamma$-ray energies. The importance of well-vetted SMBHB candidates to searches for gravitational waves is pointed out. We estimate the fraction of blazars that are SMBHB candidates to be > 1 in 100. Thus monitoring programs covering tens of thousands of blazars could discover hundreds of SMBHB candidates.