The Unanticipated Phenomenology of the Blazar PKS 2131–021: A Unique Supermassive Black Hole Binary Candidate

Creators: O'Neill, S.; Kiehlmann, S.; Readhead, A. C. S.; Aller, M. F.; Blandford, R. D.; Liodakis, I.; Lister, M. L.; Mróz, P. J.; O'Dea, C. P.; Pearson, T. J.; Ravi, V.; Vallisneri, M.; Cleary, K. A.; Graham, M. J.; Grainge, K. J. B.; Hodges, M. W.; Hovatta, T.; Lähteenmäki, A.; Lamb, J. W.; Lazio, T. J. W.; Max-Moerbeck, W.; Pavlidou, V.; Prince, T. A.; Reeves, R. A.; Tornikoski, M.; Vergara de la Parra, P.; Zensus, J. A.

Abstract

Most large galaxies host supermassive black holes in their nuclei and are subject to mergers, which can produce a supermassive black hole binary (SMBHB), and hence periodic signatures due to orbital motion. We report unique periodic radio flux density variations in the blazar PKS 2131−021, which strongly suggest an SMBHB with an orbital separation of ∼0.001–0.01 pc. Our 45.1 yr radio light curve shows two epochs of strong sinusoidal variation with the same period and phase to within ≲2% and ∼10%, respectively, straddling a 20 yr period when this variation was absent. Our simulated light curves accurately reproduce the "red noise" of this object, and Lomb–Scargle, weighted wavelet Z-transform and least-squares sine-wave analyses demonstrate conclusively, at the 4.6σ significance level, that the periodicity in this object is not due to random fluctuations in flux density. The observed period translates to 2.082 ± 0.003 yr in the rest frame at the z = 1.285 redshift of PKS 2131−021. The periodic variation in PKS 2131−021 is remarkably sinusoidal. We present a model in which orbital motion, combined with the strong Doppler boosting of the approaching relativistic jet, produces a sine-wave modulation in the flux density that easily fits the observations. Given the rapidly developing field of gravitational-wave experiments with pulsar timing arrays, closer counterparts to PKS 2131−021 and searches using the techniques we have developed are strongly motivated. These results constitute a compelling demonstration that the phenomenology, not the theory, must provide the lead in this field.

Additional Information

© 2022. 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. Received 2021 October 30; revised 2022 January 20; accepted 2022 January 21; published 2022 February 23. We thank the anonymous referee for carrying out a very detailed and thorough review of the original manuscript and for many insightful suggestions and questions, which have greatly improved this paper. 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 5 yr in the absence of agency funding. Without this private support this paper would not have been written. 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. The UMRAO program received support from NSF grants AST-8021250, AST-8301234, AST-8501093, AST-8815678, AST-9120224, AST-9421979, AST-9617032, AST-9900723, AST-0307629, AST-0607523, and earlier NSF awards, and from NASA grants NNX09AU16G, NNX10AP16G, NNX11AO13G, and NNX13AP18G. Additional funding for the operation of UMRAO was provided by the University of Michigan. The NANOGrav project receives support from National Science Foundation (NSF) Physics Frontier Center award No. 1430284. T.H. was supported by the Academy of Finland projects 317383, 320085, and 322535. S.K. acknowledges support from the European Research Council (ERC) under the European Unions Horizon 2020 research and innovation program under grant agreement No. 771282. W.M. acknowledges support from ANID projects Basal AFB-170002 and FONDECYT 11190853. : R.R. gratefully acknowledges support by the ANID BASAL projects ACE210002 and FB210003, and ANID Fondecyt 1181620. C.O. acknowledges support from the Natural Sciences and Engineering Research Council (NSERC) of Canada. M.V. and T.J.W.L. performed part of this work at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). V.P. acknowledges support from the Foundation of Research and Technology—Hellas Synergy Grants Program through project MagMASim, jointly implemented by the Institute of Astrophysics and the Institute of Applied and Computational Mathematics and by the Hellenic Foundation for Research and Innovation (H.F.R.I.) under the "First Call for H.F.R.I. Research Projects to support Faculty members and Researchers and the procurement of high-cost research equipment grant" (Project 1552 CIRCE). S.O. gratefully acknowledges the support of the Caltech Summer Undergraduate Research Fellowship program. P.VdlP. acknowledges support from Núcleo Milenio TITANs (NCN19-058).

Attached Files

Published - ONeill_2022_ApJL_926_L35.pdf

Submitted - 2111-02436.pdf

Files

ONeill_2022_ApJL_926_L35.pdf

Files (49.5 MB)

Name	Size	Download all
ONeill_2022_ApJL_926_L35.pdf md5:ef10a5db3272281277ec03b83de590ca	35.3 MB	Preview Download
2111-02436.pdf md5:69fb0703e6253bbbb49a914201208d41	14.2 MB	Preview Download

Additional details

	All versions	This version
Views	0	0
Downloads	0	0
Data volume	0 Bytes	0 Bytes