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Abstract The combined University of Michigan Radio Astronomy Observatory and Owens Valley Radio Observatory blazar monitoring programs at 14.5/15 GHz provide uninterrupted light curves of ∼ 46–50 yr duration for 83 blazars, selected from among the brightest and most rapidly flaring blazars north of declination −20°. In a search for supermassive black hole binary (SMBHB) candidates, we carried out tests for periodic variability using generalized Lomb–Scargle (GLS), weighted wavelet-Z, and sine-wave fitting analyses of this sample. We used simulations to test the effects of the power-law spectrum of the power spectral density (PSD) on our findings, and show that the irregular sampling in the observed light curves has very little effect on the GLS spectra. Apparent periodicities and putative harmonics appear in all 83 of the GLS spectra of the blazars in our sample. We tested the reality of these apparent periodicities and harmonics with simulations, and found that in the overwhelming majority of cases, they must be due to the steep slope of the PSD and the random nature of blazar flares, implying that, in general, apparent periodicities and harmonics in blazar light curves observed in any energy band should be treated with great caution. We find one new SMBHB candidate: PKS 1309+1154, which exhibits a 17.9 yr periodicity. The fraction of SMBHB candidates in our sample is $2.4_{-0.8}^{+3.2}\%$. 

Abstract We recently discovered a supermassive black hole binary (SMBHB) candidate, PKS J1309+1154, in the combined 46 yr University of Michigan Radio Astronomy Observatory (UMRAO) plus Owens Valley Radio Observatory (OVRO) blazar monitoring programs at 14.5/15 GHz. The light curve of PKS 1309+1154 exhibits a 17.9 yr periodicity. We also reported a hint of a first harmonic with a 9 yr periodicity in this object. Further analysis of the PKS J1309+1154 light curve provides compelling evidence that both the fundamental and the harmonic are real, confirming the existence of real periodicities in blazar light curves. This is the first case, to our knowledge, of watertight evidence for a fundamental periodicity and a harmonic periodicity in a blazar light curve. It makes PKS J1309+1154 astrongSMBHB candidate and thus the third such candidate to be revealed through long-term radio monitoring, the other two being PKS J0805–0111 and PKS 2131–021, both discovered through the OVRO 40 m Telescope monitoring program. It is argued that hundreds of SMBHB candidates will be discovered by the combination of the South Pole Telescope, the Vera Rubin Observatory, and the Simons Observatory. Coherent searches for gravitational waves from a network of SMBHB candidates, starting immediately, are strongly motivated. 

Abstract This paper addresses, for the first time, a key aspect of the phenomenology of compact symmetric objects (CSOs): the characteristics of their radio spectra. We present a radio-spectrum description of a complete sample of high-luminosity CSOs (CSO-2s), which shows that they exhibit the complete range of spectral types, including flat-spectrum sources (α≥ −0.5), steep-spectrum sources (α< −0.5), and peaked-spectrum sources. We show that there is no clear correlation between spectral type and size, but there is a correlation between the high-frequency spectral index and both object type and size. We also show that, to avoid biasing the data and to understand the various classes of active galactic nuclei (AGN) involved, the complete range of spectral types should be included in studying the general phenomenology of compact jetted AGN, and that complete samples must be used, selected over a wide range of frequencies. We discuss examples that demonstrate these points. We find that the high-frequency spectral indices of CSO-2s span −1.3 <α_hi< −0.3 and hence that radio spectral signatures cannot be used to discriminate definitively between CSO-2s, binary galactic nuclei, and millilensed objects, unless they haveα_hi> −0.3. 

Abstract Owens Valley Radio Observatory observations of the supermassive black hole binary (SMBHB) candidate PKS 2131−021 revealed, for the first time, six likely characteristics of the phenomenology exhibited by SMBHBs in blazars, of which the most unexpected and critical is sinusoidal flux density variations. We have now identified a second blazar, PKS J0805−0111, showing similar variations, with a period of 1.422 ± 0.005 yr in the rest frame of thez= 1.388 object. PKS J0805−0111 displays five of the six characteristics observed in PKS 2131−021. To estimate the significance of the sinusoidal variations, we generate 10⁶simulated light curves that reproduce the radio variability characteristics of PKS J0805−0111 and show that the global probability that the periodicity we detect is due to the red-noise tail of the power spectral density isp= 6.7 × 10⁻⁵(3.82σ). This shows that PKS 2131−021 is not a unique case. The discovery of these two objects in a sample of 1158 blazars allows us to reject, at ap-value ∼0.003, the null hypothesis that the sinusoidal variations in these two blazars are all due to a red-noise process. We estimate that the number of SMBHB candidates among blazars is ∼1 in 100.