A Search for Supermassive Black Hole Binary Candidates in 46 yr Radio Light Curves of 83 Blazars

The combined University of Michigan Radio Astronomy Observatory and Owens Valley Radio Observatory blazar monitoring programs at $14.5/15,\mathrm{GHz}$ provide uninterrupted light curves of $\sim 46$--$50,\mathrm{yr}$ duration for 83 blazars, selected from among the brightest and most rapidly flaring blazars north of declination $-20^\circ$. 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 $17.9,\mathrm{yr}$ periodicity. The fraction of SMBHB candidates in our sample is $2.4_{-0.8}^{+3.2}%$.