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.
The identification of supermassive black hole binaries (SMBHBs) will open the field to multimessenger astronomy through the gravitational radiation they produce. Pulsar timing arrays provide a powerful technique for searching for nanohertz signals from gravitational waves from SMBHBs through the timing of millisecond pulsars (Holgado et al. 2018; Burke-Spolaor et al. 2019). However, in spite of the fact that galaxy mergers are not uncommon, there are relatively few instances of two galaxies with supermassive black holes (SMBHs) in their nuclei being seen in the actual process of the merging of the spheres of influence of their SMBHs, or of the following stages, when an SMBHB forms by ejecting stars from the merging central clusters, spirals in more closely owing to gravitational radiation, and finally coalesces (Begelman et al. 1980). A particularly fine example of the early stage of possible evolution toward an SMBHB is that of 3C 75 (Owen et al. 1985), where both SMBHs are producing radio jets, and their projected separation is 7.2 kpc. On parsec scales the best SMBHB candidate is B3 0402+379 (Rodriguez et al. 2006; Bansal et al. 2017), with a projected separation of 7.3 pc, a deduced period of 3 × 104 yr, and a deduced SMBHB mass of ≈1.5 × 1010 M⊙. The strongest SMBHB candidate with a separation of ≪1 pc is OJ 287 (Sillanpaa et al. 1988; Valtonen et al. 2016; Dey et al. 2021), for which the separation is ∼0.1 pc, the deduced primary mass is ≈1.8 × 1010 M⊙, and the deduced secondary mass is ≈1.5 × 108 M⊙. At separations ≪1 pc, even with high-frequency very long baseline interferometry (VLBI), for all but the closest active galactic nuclei (AGNs), we lack the angular resolution required to demonstrate the existence of an SMBHB through imaging, and we have to look for other signatures.