The Atacama Cosmology Telescope: Observations of supermassive black hole binary candidates Strong sinusoidal variations at 95, 147, and 225 GHz in PKS 2131–021 and PKS J0805–011

Large sinusoidal variations in the radio light curves of the blazars PKS J0805–0111 and PKS 2131–021 have recently been discovered with an 18-year monitoring programme at the Owens Valley Radio Observatory, making these systems strong supermassive black hole binary (SMBHB) candidates. The sinusoidal variations in PKS 2131–021 dominate its light curves from 2.7 GHz to optical frequencies. We report sinusoidal variations observed in both objects with the Atacama Cosmology Telescope (ACT) at 95, 147, and 225 GHz consistent with the radio light curves. The ACT 95 GHz light curve of PKS 2131–021 agrees well with the contemporaneous 91.5 GHz ALMA light curve and is comparable in quality, while the ACT light curves of PKS J0805–0111, for which there are no ALMA or other millimetre light curves, show that PKS 2131–021 is not an isolated case, and that this class of AGN exhibits the following properties: (a) the sinusoidal pattern dominates over a broad range of frequencies; (b) the amplitude of the sine wave compared to its mean value is monochromatic (i.e. nearly constant across frequencies); (c) the phase of the sinusoid phase changes monotonically as a function of frequency; (d) the sinusoidal variations are intermittent. We describe a physical model for SMBHB systems, the modified Kinetic Orbital model, that explains all four of these phenomena. The monitoring of ∼8000 blazars by the Simons Observatory over the next decade should provide a large number of SMBHB candidates that will shed light on the nature of the nanohertz gravitational-wave background.