Astrophysicists of the SAO RAS studied the variability of emission of the blazar AO 0235+164 over a 27-year period from 1997 to 2023. For this purpose, multi-wavelength data obtained with the RATAN-600 radio telescope and meter-class optical telescopes of the SAO RAS were used along with the data from the radio telescopes of the CrAO RAS and IAA RAS, SMA facility (USA) and the Fermi-LAT space mission. The total radio spectrum is shown in Fig 1. Blazars are extreme representatives of active galactic nuclei, the most powerful sources in the Universe. They are characterized by the presence of a jet, launching from the center of the galaxies that is directed along the line of sight of the observer on Earth and is enhanced due to relativistic beaming effects across the entire electromagnetic spectrum.

    For the analysis, the light curves are divided into 4 epochs, each associated with one major flare (shown in Fig. 2) and the epoch of quiescent state in 2009-2014 without flares. Correlations between the data in the radio, optical, and γ-ray bands are found in all 4 epochs of activity, and the delay time of flares increases linearly with increasing wavelength reaching 1.7 years for the radio data in the centimeter range. This means that the flare starts and develops at high frequencies, with subsequent propagation to lower frequencies. During the period of low activity of AO 0235+164 in 2009-2014, the variability in the optical and γ-ray bands reaches 30-50%, which may be caused by the interaction of shock wave remnants that arose during previous powerful outbursts. The correlation between brightness variations in all the studied ranges for this period, which was discovered for the first time, may mean that the mechanisms causing the variability of radiation in the low state are similar to the mechanisms that occur in the active phases and are accompanied by a multiple enhancement in brightness.

    The frequency method of searching for hidden variability in the 27-year blazar light curves, taking into account the inhomogeneity of the data and the influence of low-frequency (and so-called “red”) noise inherent in the experimental data, has revealed the presence of significant 6-year quasi-periods in all spectral ranges studied. For the epoch of the quiescent state, significant quasi-periodicity was discovered for the first time on a timescale of about 2 years. It is suggested that the 6-year quasi-period may reflect the time between the most prominent outbursts in the light curves, which may be stochastic in nature, and the detected quasi-periodicity may not be related to the characteristics of the active nucleus and the relativistic jet. At the same time, we believe that the periodicity detected for the low state reflects the basic properties of the AGN in AO 0235+164.

    This study expands our understanding of the mechanisms that cause the variability of blazar radiation and may help us further explore these unique sources of electromagnetic radiation.

Fig. 1. Broad-band radio spectrum of AO 0235+164. The measurements from RATAN-600, RT-32, RT-22, and SMA obtained in 1997-2023 and colored by pink as well as the measurements from the CATS database (grey points). The orange line is the result of fitting the model spectrum to the average total spectrum. This model spectrum is the sum of two components: the variable High Frequency Component and the non-variable Low Frequency Component.

Fig. 2. Multiband light curves of AO 0235+164 in 1997-2023. Four epochs chosen for analysis are depicted by the straight vertical lines. The historically quiet state between 2009 November and 2014 March is indicated by the grey area.

Published:
Vlasyuk V. V. et al., Multiwavelength variability of the blazar AO 0235+16, MNRAS, 2024, Volume 535, Issue 3, Pages 2775-2799 (https://academic.oup.com/mnras/article/535/3/2775/7881583)