Blazars are among the brightest and most variable objects in the Universe. They are active galaxies hosting a supermassive black hole at their center. Narrow jets of matter, known as relativistic jets, erupt from regions near the black hole, traveling almost at the speed of light. If such a jet is directed toward Earth, the object appears especially bright and is observed as a blazar. This orientation explains the highly variable emission of blazars across the entire electromagnetic spectrum, from radio waves to gamma rays.

Fig. 1. An artist’s impression of a supermassive binary black hole system (SMBBH). © 2026 E. Chmyreva / SAO RAS

    One important feature of some blazars is the presence of recurring changes in brightness. The quasi-periodic variability occurs at the timescales ranging from months to many years. Possible causes for this regularity include the presence of a supermassive binary black hole (SMBBH) system (Fig. 1) orbiting each other, a slow change in the direction of the relativistic jet, or a complex internal structure of the jet near its base. Understanding the nature of these processes requires comparing observational data in different spectral ranges — from radio waves to γ-rays.

    The blazar Ton 599 is a bright radio quasar with a flat spectrum and is known for its powerful flares and detection in the high-energy γ-ray emission (TeV). Previous studies have revealed a quasi-periodic variability of its synchrotron emission at the timescales of several years. Our study analyzed its behavior using observations from several radio and optical telescopes, such as RATAN-600 and Zeiss-1000 (SAO RAS), RT-22 (CrAO RAS), and RT-32 (IAA RAS), as well as γ-ray measurements from the Fermi-LAT space observatory and Submillimeter Array (SMA) measurements (Fig. 2). As a result, we studied the multiwavelength behavior of Ton 599 over a time interval of about 40 years, from 1983 to 2025.

Fig. 2. Multiband light curves of Ton 599 in 1997-2025.

    The γ-ray, optical, and radio emissions are found to be highly correlated with emission at longer wavelengths lagging behind that at shorter wavelengths. The delay can be as long as one year. This pattern indicates that the emission is generated by the same population of charged particles moving at near-light speeds in the strong magnetic field of the relativistic jet.

    Using the Weighted Wavelet Z-transform (WWZ) we found the presence of several periodic components with characteristic variability timescales ranging from 1.4 to 7.5 years. To interpret the observed data, we consider a model of SNBBH in the center of Ton 599 with a total mass of 5×10⁸ⵙ (Fig. 3). Our calculations show that the observed multiwavelength behavior of Ton 599 can be well described by a system with an orbital period of 1.2-1.7 years and a jet precession period of 5.8-7.7 years (Fig. 4). Faster variability on timescales of days and weeks, as well as powerful short-term outbursts, are likely associated with compact emission regions within the jet, arising in shockwave zones where additional particle acceleration occurs. Thus, the complex multiwavelength variability of Ton 599 is likely explained by a combination of geometric effects associated with changes in the jet's orientation relative to the observer and rapid physical processes occurring within the jet.

Fig. 3. A schematic illustration of jet orbital motion and precession. The elements are not to scale. The binary SMBH is shown as two black dots, the jet is the red arrow, the angle between it and the observer’s line of sight is θ, the orbit inclination angle with respect to the observer is i, and the precessing cone angle is Ω.

Fig. 4. The jet orbital motion and precession model (orange) applied to the 5 GHz light curve of Ton 599.

Published:
Sotnikova Yu. et al., MNRAS, March 2026, DOI:10.1093/mnras/stag333, arXiv:2603.05894