Atacama Cosmology Telescope: Observations of supermassive black hole binary candidates. Strong sinusoidal variations at 95, 147 and 225 GHz in PKS 2131$-$021 and PKS J0805$-$0111

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. Monitoring of ${\sim}8000$ 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.

💡 Research Summary

The paper presents new millimetre‑wave observations of two blazars, PKS 2131‑021 and PKS J0805‑0111, that have previously been identified as strong supermassive black‑hole binary (SMBHB) candidates based on 18‑year, high‑cadence monitoring at 15 GHz with the Owens Valley Radio Observatory (OVRO). Those earlier studies revealed large‑amplitude, sinusoidal variations that dominate the light curves of both sources, with PKS 2131‑021 showing the same sinusoid from 2.7 GHz up to the optical band.

Using data from the Atacama Cosmology Telescope (ACT) taken between 2016 and 2022 at three frequencies (95, 147 and 225 GHz), the authors independently confirm the sinusoidal behaviour. The ACT 95 GHz light curve of PKS 2131‑021 matches contemporaneous ALMA measurements at 91.5 GHz in both amplitude and phase, demonstrating that ACT can deliver millimetre‑wave blazar monitoring of comparable quality to dedicated interferometers. For PKS J0805‑0111, which lacks any previous mm‑wave coverage, ACT provides the first detection of the same sinusoidal pattern, establishing that the phenomenon is not unique to PKS 2131‑021.

Four key phenomenological properties are highlighted: (a) the sinusoid dominates over a very broad frequency range; (b) the ratio of sinusoid amplitude to the mean flux (the “monochromaticity”) is essentially constant across frequencies; (c) the phase of the sinusoid shifts monotonically to earlier times as frequency increases, consistent with an optical‑depth effect that probes progressively deeper regions of the jet; and (d) the sinusoidal signal is intermittent, appearing and disappearing over multi‑year intervals.

To explain these observations the authors revisit the Kinetic Orbital (KO) model, in which one black hole in the binary launches a relativistic jet whose direction is periodically aberrated by the orbital motion, producing a sinusoidal modulation in the observed flux. The original KO model, however, struggles to maintain a coherent helical jet over many orbital periods. The paper therefore introduces a Modified Kinetic Orbital (MKO) model. In MKO a sub‑relativistic wind, dragged by the binary orbit, confines the relativistic jet, allowing a helical structure to be produced with only a few windings. The wind also naturally creates intermittent jet interruptions, accounting for the observed on‑off behaviour of the sinusoid. Recent three‑dimensional magnetohydrodynamic simulations of binary accretion flows are cited as supporting evidence for such wind‑jet interactions.

Methodologically, the authors fit a sinusoid to each light curve by maximizing a likelihood that includes an offset, amplitude, phase, and an extra term (ξ) to capture correlated noise. The period P is first determined from the OVRO data restricted to the time span where ACT data exist (≈ 6 yr) to avoid contamination from longer‑term jet variability that would act as additional noise. With P fixed, a joint fit across all frequencies yields precise phase measurements. The phase shift is found to be roughly linear with frequency, amounting to about 0.2 rad per GHz, while the amplitude‑to‑mean ratio remains at ~0.07 ± 0.01 across the three ACT bands and the ALMA band.

Finally, the paper looks ahead to the Simons Observatory and other next‑generation CMB/millimetre surveys. With an expected monitoring of ~8000 blazars over the next decade, the authors argue that hundreds to thousands of SMBHB candidates could be identified via their sinusoidal signatures. Such a population would dramatically improve constraints on the nanohertz gravitational‑wave background measured by pulsar timing arrays, and would open a new avenue for multi‑messenger astrophysics that combines electromagnetic monitoring with gravitational‑wave observations.