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

S. O’neill, S. Kiehlmann, A. C. S. Readhead, M. F. Aller, R. D. Blandford, I. Liodakis, M. L. Lister, P. Mróz, C. P. O’dea, T. J. Pearson, V. Ravi, M. Vallisneri, K. A. Cleary, M. J. Graham, K. J. B. Grainge, M. W. Hodges, T. Hovatta, A. Lähteenmäki, J.W. Lamb, T. J. W. LazioW. Max-moerbeck, V. Pavlidou, T. A. Prince, R. A. Reeves, M. Tornikoski, P. Vergara De La Parra, J. A. Zensus

Research output: Contribution to journalArticlepeer-review


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-year 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-year 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 years 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 which 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 resultsconstitute a compelling demonstration that the phenomenology, not the theory, must provide the lead in this field.

Keywords: galaxies: active, galaxies: jets, galaxies: BL Lacertae objects: individual (PKS 2131−021)
Original languageEnglish
Pages (from-to)L35
JournalThe Astrophysical Journal Letters
Issue number2
Early online date23 Feb 2022
Publication statusPublished - 23 Feb 2022


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