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Published May 20, 2025 | Version Published
Journal Article Open

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

Creators

  • 1. ROR icon Foundation for Research and Technology Hellas
  • 2. ROR icon University of Concepción
  • 3. ROR icon Kavli Institute for Particle Astrophysics and Cosmology
  • 4. ROR icon University of Crete
  • 5. ROR icon Marshall Space Flight Center
  • 6. ROR icon University of Warsaw
  • 7. ROR icon University of Oslo
  • 8. ROR icon California Institute of Technology
  • 9. JILA, University of Colorado and National Institute of Standards and Technology, 440 UCB, Boulder, CO 80309-0440, USA
  • 10. ROR icon National Institute of Standards and Technology
  • 11. ROR icon University of Colorado Boulder
  • 12. ROR icon University of Cambridge
  • 13. ROR icon University College London
  • 14. ROR icon Denison University
  • 15. ROR icon University of Turku
  • 16. ROR icon Aalto University
  • 17. ROR icon Purdue University West Lafayette
  • 18. ROR icon University of Chile
  • 19. ROR icon University of Manitoba
  • 20. ROR icon University of Alabama in Huntsville
  • 21. ROR icon University of Michigan–Ann Arbor
  • 22. ROR icon Jet Propulsion Lab
  • 23. ROR icon Princeton University
  • 24. ROR icon Max Planck Institute for Radio Astronomy

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 P15 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 Poptical = (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.

Copyright and License

© 2025. The Author(s). Published by the American Astronomical Society.

Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Acknowledgement

This work is supported by NSF grants AST2407603 and AST2407604. We thank the California Institute of Technology and the Max Planck Institute for Radio Astronomy for supporting the OVRO 40 m program under extremely difficult circumstances over the last 8 yr in the absence of agency funding. Without this private support, these observations could not have been made. We thank Adam Hincks for useful discussions. We also thank all the volunteers who have enabled this work to be carried out. Prior to 2016, the OVRO program was supported by NASA grants NNG06GG1G, NNX08AW31G, NNX11A043G, and NNX13AQ89G from 2006 to 2016 and NSF grants AST-0808050 and AST-1109911 from 2008 to 2014. S.K. and K.T. acknowledge support from the European Research Council (ERC) under the European Unions Horizon 2020 research and innovation program under grant agreement No. 771282. I.L and S.K. were funded by the European Union ERC-2022-STG—BOOTES—101076343. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Council Executive Agency. Neither the European Union nor the granting authority can be held responsible for them. W.M. acknowledges support from ANID project Basal FB210003. A.S. and R.B acknowledge support by a grant from the Simons Foundation (00001470,RB,AS). R.R. and B.M. and P.V.d.l.P. acknowledge support from ANID Basal AFB-170002, from Núcleo Milenio TITANs (NCN2023_002), and CATA BASAL FB210003. P.V.d.l.P. also acknowledges support by the National Agency for Research and Development (ANID)/Scholarship Program/Doctorado Nacional/2023–21232103. C.O. acknowledges support from the Natural Sciences and Engineering Research Council (NSERC) of Canada. T.H. acknowledges support from the Academy of Finland projects 317383, 320085, 322535, and 345899. The NANOGrav collaboration receives support from National Science Foundation (NSF) Physics Frontiers Center award Nos. 1430284 and 2020265. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2011.0.00001.CAL. ALMA is a partnership of ESO (representing its member states), NSF (USA), and NINS (Japan), together with NRC (Canada), NSTC and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This research has made use of the NASA/IPAC Extragalactic Database (NED), which is funded by the National Aeronautics and Space Administration and operated by the California Institute of Technology.

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Additional details

Related works

Is new version of
Discussion Paper: arXiv:2407.09647 (arXiv)

Funding

National Science Foundation
AST2407603
National Science Foundation
AST2407604
National Aeronautics and Space Administration
NNG06GG1G
National Aeronautics and Space Administration
NNX08AW31G
National Aeronautics and Space Administration
NNX11A043G
National Aeronautics and Space Administration
NNX13AQ89G
National Science Foundation
AST-0808050
National Science Foundation
AST-1109911
European Research Council
771282
European Research Council
101076343
Agencia Nacional de Investigación y Desarrollo
FB210003
Simons Foundation
00001470
Agencia Nacional de Investigación y Desarrollo
AFB-170002
Millenium Nucleus for Planet Formation
NCN2023_002
Agencia Nacional de Investigación y Desarrollo
2023-21232103
Natural Sciences and Engineering Research Council
Research Council of Finland
317383
Research Council of Finland
320085
Research Council of Finland
322535
Research Council of Finland
345899
National Science Foundation
1430284
National Science Foundation
2020265
Jet Propulsion Laboratory

Dates

Accepted
2025-03-24
Available
2025-05-14
Published

Caltech Custom Metadata

Caltech groups
Astronomy Department, LIGO, Owens Valley Radio Observatory, TAPIR, Walter Burke Institute for Theoretical Physics, Division of Physics, Mathematics and Astronomy (PMA)
Publication Status
Published