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 J0805–0111

¹ David A. Dunlap Department of Astronomy & Astrophysics, University of Toronto 50 St. George St. Toronto ON M5S 3H4, Canada
² Specola Vaticana (Vatican Observatory) V-00120 Vatican City, Vatican City State
³ Kavli Institute for Astronomy & Astrophysics, Peking University Beijing 100871, People’s Republic of China
⁴ Department of Astronomy, School of Physics, Peking University Beijing 100871, People’s Republic of China
⁵ Astronomical Observatory, University of Warsaw Al. Ujazdowskie 4 00-478 Warszawa, Poland
⁶ Institute of Theoretical Astrophysics University of Oslo, Norway
⁷ Institute of Astrophysics, Foundation for Research & Technology-Hellas GR-71110 Heraklion, Greece
⁸ Kavli Institute for Particle Astrophysics and Cosmology, Department of Physics, Stanford University Stanford CA 94305, USA
⁹ Canadian Institute for Theoretical Astrophysics, University of Toronto 60 St. George Street Toronto ON M5S 3H8, Canada
¹⁰ Department of Physics & Astronomy, University of Pennsylvania 209 South 33rd Street Philadelphia PA 19104, USA
¹¹ Joseph Henry Laboratories of Physics, Jadwin Hall, Princeton University Princeton NJ 08544, USA
¹² Department of Astrophysical Sciences, Peyton Hall, Princeton University Princeton NJ 08544, USA
¹³ Division of Physics, Mathematics, and Astronomy, California Institute of Technology Pasadena CA 91125, USA
¹⁴ Dunlap Institute for Astronomy & Astrophysics, University of Toronto 50 St. George St. Toronto ON M5S 3H4, Canada
¹⁵ Instituto de Astrofísica & Centro de Astro-Ingeniería, Facultad de Física, Pontificia Universidad Católica de Chile Av. Vicuña Mackenna 4860 7820436 Macul Santiago, Chile
¹⁶ Department of Astronomy & Astrophysics, University of Chicago 5640 South Ellis Avenue Chicago IL 60637, USA
¹⁷ Department of Physics and Astronomy, University of Pittsburgh 3941 O’Hara Street Pittsburgh PA 15260, USA
¹⁸ TAPIR, Mailcode 350-17, California Institute of Technology Pasadena CA 91125, USA
¹⁹ Walter Burke Institute for Theoretical Physics, California Institute of Technology Pasadena CA 91125, USA
²⁰ Department of Physics, Cornell University Ithaca NY 14853, USA
²¹ Department of Astronomy, Cornell University Ithaca NY 14853, USA
²² Department of Physics & Astronomy, Haverford College Haverford PA 19041, USA
²³ Owens Valley Radio Observatory, California Institute of Technology Pasadena CA 91125, USA
²⁴ Instituto de Física, Pontificia Universidad Católica de Valparaíso Casilla 4059 Valparaíso, Chile
²⁵ Mitchell Institute for Fundamental Physics & Astronomy and Department of Physics & Astronomy, Texas A&M University College Station TX 77843, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 4 April 2025
Accepted: 8 December 2025

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.