The Atacama Cosmology Telescope: Observations of supermassive black hole binary candidates

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.

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Journal
Astronomy and Astrophysics
Published
2025-12-19
DOI
https://doi.org/10.1051/0004-6361/202555047
Primary Topic
Astrophysics and Cosmic Phenomena
Type
article
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article

The Atacama Cosmology Telescope: Observations of supermassive black hole binary candidates

Sigurd Næss, Elias R. Most, Carlos Hervías-Caimapo, R. D. Blandford et al.
Astronomy and Astrophysics
Astrophysics and Cosmic Phenomena
article

The Atacama Cosmology Telescope: Observations of supermassive black hole binary candidates

Sigurd Næss, Elias R. Most, Carlos Hervías-Caimapo, R. D. Blandford, M. J. Graham, Michelle Devlin, S. Kiehlmann, A. Foster, Xiaoyi Ma, John Orlowski-Scherer, Aretaios Lalakos, Suzanne T. Staggs, Arthur Kosowsky, Andrew G. Sullivan, J. C. Hood, P. Mróz, Cristobál Sifón, A. C. S. Readhead, Bruce Partridge, Yilun Guan, Cristian Vargas, J. Richard Bond, Lyman A. Page, Jo Dunkley, Michael D. Niemack, Adam D. Hincks
article en

Abstract

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.

Astronomy and AstrophysicsVol. 706
King University (US), California Institute of Technology (US), Pontificia Universidad Católica de Valparaíso (CL), Haverford College (US), Pontificia Universidad Católica de Chile (CL), University of Pittsburgh (US), University of Toronto (CA), Princeton University (US), Peking University (CN), Cornell University (US), Vatican Secret Archives (VA), Mitchell Institute (US), University of Chicago (US), Kavli Institute for Particle Astrophysics and Cosmology (US), Architecture Technology Corporation (United States) (US), Canadian Institute for Theoretical Astrophysics (CA), Centre for Theoretical Physics and Astrophysics (UZ), Burke Foundation (US), Astronomical Observatory (RS), University of Warsaw (PL), University of Pennsylvania (US), Foundation for Research and Technology Hellas (GR)
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