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Data from long-term multi-frequency monitoring are used to analyze variations in the flux density of the active galactic nucleus S4 0954+658. These data were obtained at the CrimeanAstrophysical Observatory, the Metsahovi Radio Observatory of Aalto University, the University of Michigan Radio Astronomy Observatory, the Cavendish Laboratory of Cambridge University, the Special Astrophysical Observatory, and the National Institute of Astrophysics, Optics, and Electronics; 0.1–300-GeV data from the Fermi space gamma-ray observatory were also used. Radio data at 4.8, 8, 14.5, 15, 22.2, and 36.8 GHz are considered together with optical and near-infrared data in the R, J, H, and K filters.

Long series of multi-frequency monitoring data of the active galactic nucleus (AGN) AO 0235+164 from the radio to the gamma-ray are analyzed. AO 0235+164 may be a close binary system comprised of two supermassive black holes (SMBHs). A harmonic analysis has established the presence of orbital and precessional periods of two and eight years, respectively. These values are close to the analogous periods found in other AGNs that have been proposed to be close SMBH binary systems in a stage of their evolution close to coalescence. A cross-correlation analysis is used to find the time delay between corresponding flares occurring in different wavebands. The empirical dependence found earlier for a number of other AGNS relating the delay in the flux variations with the frequency is observed for this object, and has the form of an inverse logarithmic dependence, providing evidence that the flux variability of AGNs at centimeter and shorter wavelengths is intrinsic. The standard picture of jet activity of AGNs supposes plasma formations that move from the central regions along outflows, becoming optically thin at longer and longer wavelengths as they do so. The physical and dynamical characteristics of AO 0235+164 suggest it is a close binary with SMBHs with similar masses of the order of 1010M⊙. This is one of the most massive systems of two SMBHs. The velocities of the companion and central black hole about their common center of mass are estimated to be 104 and 5×103 km/s. The results obtained suggest that observed bright AGNs form a relatiavely rare population among massive elliptical galaxies, due to their narrowly directed emission and their short lifetimes.

Flux-density variations of the quasar S0528+134 (Nimfa) are analyzed based on long-term monitoring at five radio frequencies between 4.8 and 37 GHz, performed at the Crimean Astrophysical Observatory, the Metsahovi Radio Observatory of Aalto University, and the University of Michigan Radio Astronomy Observatory. The dynamics of a powerful flare in 1996 are analyzed using gamma-ray (0.1–300 GeV), X-ray (2–10 keV, 15–50 keV), and radio observations. The delays of the flare between different spectral ranges and between different radio wavelengths have been measured. The dependence for the delays at different radio wavelengths relative to the X-ray and optical flares is established based on long-term observations in the X-ray, optical, and radio obtained from 2004 to 2013. Multi-frequency monitoring in the radio is used to estimate the orbital and precession periods in the binary supermassive black hole system S0528+134 and the physical characteristics of this system.

Long-term, multi-frequency optical and radio monitoring data for the Active Galactic Nuclei (AGNs) AO 0235+164 and S5 0716+714 are used to derive the characteristics of their radiation on time scales from hours to decades. Structure-function and harmonic analyses are carried out to determine the parameters of the radiation of these AGNs and explain their physical characteristics. Periodic components that could be associated with the motion of companions along an orbit in a close binary system of two supermassive black holes are identified. Both of the analysis methods used yield consistent results for the harmonic components. The results obtained are consistent with the idea that we are viewing systems of two supermassive black holes in a stage of their evolution that is close to coalescence. The substantial energy release of these AGNs can be interpreted as dynamical losses arising during the motion of the companion about the common center of gravity in a dense accreting medium, sometimes crossing through the accretion disk.