PKS 2131−021—Discovery of Strong Coherent Sinusoidal Variations from Radio to Optical Frequencies: Compelling Evidence for a Blazar Supermassive Black Hole Binary
Abstract Haystack and Owens Valley Radio Observatory observations recently revealed strong, intermittent, sinusoidal total flux-density variations that maintained their 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 a coherence over 47.9 yr, with an observed period P 15 GHz = (1739.8 ± 17.4) days. We reject, with p -value = 2.09 × 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 optical = (1764 ± 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 γ -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.
Authors
- T. J. Pearson (ORCID: https://orcid.org/0000-0001-5213-6231)
- Sigurd Næss (ORCID: https://orcid.org/0000-0002-4478-7111)
- Vikram Ravi (ORCID: https://orcid.org/0000-0002-7252-5485)
- D. C. Homan (ORCID: https://orcid.org/0000-0002-4431-0890)
- M. L. Lister (ORCID: https://orcid.org/0000-0003-1315-3412)
- R. Maiolino (ORCID: https://orcid.org/0000-0002-4985-3819)
- Ioannis Liodakis (ORCID: https://orcid.org/0000-0001-9200-4006)
- R. Reeves (ORCID: https://orcid.org/0000-0001-5704-271X)
- Yuanze Ding (ORCID: https://orcid.org/0000-0002-5770-2666)
- W. Max-Moerbeck (ORCID: https://orcid.org/0000-0002-5491-5244)
- C. P. O’Dea (ORCID: https://orcid.org/0000-0001-6421-054X)
- R. D. Blandford (ORCID: https://orcid.org/0000-0002-1854-5506)
- Michele Vallisneri (ORCID: https://orcid.org/0000-0002-4162-0033)
- M. J. Graham (ORCID: https://orcid.org/0000-0002-3168-0139)
- J. A. Zensus (ORCID: https://orcid.org/0000-0001-7470-3321)
- Mitchell C. Begelman (ORCID: https://orcid.org/0000-0003-0936-8488)
- S. Kiehlmann (ORCID: https://orcid.org/0000-0001-6314-9177)
- Katerina Chatziioannou (ORCID: https://orcid.org/0000-0002-5833-413X)
- S. R. Kulkarni (ORCID: https://orcid.org/0000-0001-5390-8563)
- V. Pavlidou (ORCID: https://orcid.org/0000-0002-0870-1368)
- Andrew G. Sullivan (ORCID: https://orcid.org/0000-0002-9545-7286)
- P. Mróz (ORCID: https://orcid.org/0000-0001-7016-1692)
- A. C. S. Readhead (ORCID: https://orcid.org/0000-0001-9152-961X)
- Konstantinos Tassis (ORCID: https://orcid.org/0000-0002-8831-2038)
- Brian Molina (ORCID: https://orcid.org/0009-0000-9963-6874)
- S. O’Neill (ORCID: https://orcid.org/0000-0003-2454-6828)
- Thomas A. Prince (ORCID: https://orcid.org/0000-0002-8850-3627)
- Fiona Harrison (ORCID: https://orcid.org/0000-0002-4226-8959)
- P. V. de la Parra (ORCID: https://orcid.org/0000-0001-5957-1412)
- M. F. Aller (ORCID: https://orcid.org/0000-0003-2483-2103)
- T. Hovatta (ORCID: https://orcid.org/0000-0002-2024-8199)
- A. Synani (ORCID: https://orcid.org/0009-0004-2614-830X)
- C. R. Lawrence (ORCID: https://orcid.org/0000-0002-5983-6481)
- T. Joseph W. Lazio
Publication Details
- Journal
- The Astrophysical Journal
- Published
- 2025-05-14
- DOI
- https://doi.org/10.3847/1538-4357/adc567
- Citations
- 11
- Primary Topic
- Astrophysics and Cosmic Phenomena
- Type
- article
- Field-Weighted Citation Impact
- 20.57
Funders
- Simons Foundation
- Academy of Finland
- Agencia Nacional de Investigación y Desarrollo