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

TL;DR

This study analyzes 46–50 year radio light curves for 83 bright blazars to search for periodic variability indicative of SMBHBs, employing GLS, WWZ, and SWF analyses and testing significance with PSD-matched simulations. It finds that most apparent periodicities and harmonics are artifacts of the steep PSD rather than true orbital signals, with PKS J1309+1154 emerging as the only plausible SMBHB candidate (P ≈ 17.9 years) though not meeting a stringent 3σ global criterion. Multi-wavelength follow-up (VLBI, polarization, ALMA, optical/IR, and X-ray) reveals coherent radio modulations tied to core/jet components and hints of frequency-dependent phase shifts, supporting but not proving a SMBHB interpretation for PKS J1309+1154. The inferred SMBHB incidence among these bright blazars is about 2.4% with substantial uncertainty, suggesting SMBHBs are present but rare in this population and emphasizing the need for longer baselines and cross-band confirmation.

Abstract

The combined University of Michigan Radio Astronomy Observatory (UMRAO) and Owens Valley Radio Observatory (OVRO) blazar monitoring programs at 14.5/15 GHz provide uninterrupted light curves of $\sim~46-50$ yr duration for 83 blazars, selected from amongst 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 (WWZ), and sine-wave fitting (SWF) 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 are due to the steep slope of the PSD, and should therefore be treated with great caution. We find one new SMBHB candidate: PKS 1309+1154, which exhibits a 17.9 year periodicity. The fraction of SMBHB candidates in our sample is $2.4_{-0.8}^{+3.2}\%$.

Figures (12)

• Figure 1: PKS J1309+1154: (a) The combined UMRAO and OVRO light curve. Red and blue symbols denote the UMRAO and OVRO data, respectively. The fitted black sine wave includes a linear downward trend and has period of 6551 days. (b) The GLS periodogram: red, blue, and green curves are the UMRAO, OVRO, and combined UMRAO+OVRO spectra, respectively. (c) The SWF power spectrum. (d) The WWZ wavelet analysis showing only one prominent feature over 46 years: color bar: WWZ power $Z$ (unitless, linear).
• Figure 2: Residual light curve of PKS J1309+1154. The light gray curve is the least squares sine wave fit to the residual light curve. The vertical dashed lines indicate the times of the maxima in the sinusoidal curve in Fig. \ref{['plt:lightcurve1']}(a).
• Figure 3: VLBA 15 GHz MOJAVE maps of (a) PKS 2131--021, and (b) PKS J1309+1154, showing the close juxtaposition of the stationary component #5 and the core. In both cases the sinusoidal variations originate not only in the core, but also in component #5.
• Figure 4: UMRAO+OVRO monitoring and MOJAVE VLBA results on PKS J1309+1154 and PKS 2131--021. (a) PKS 2131--021 adapted from Paper 1. (b) PKS J1309+1154 from MOJAVE. In both cases the combined flux densities of the core component and component #5 follow the total flux density and are clearly the major contributors to the sinusoidal variation. It is just coincidental that in both sources the stationary component is #5.
• Figure 5: The polarization of PKS J1309+1154 at 14.5 GHz from the UMRAO monitoring survey. (a) the fractional linear polarization. (b) the electric vector position angle (EVPA). The mean EVPA is aligned with the inner jet axis.