The complex variability of blazars: time-scales and periodicity analysis in S4 0954+65
Raiteri, C.M.; Villata, M.; Larionov, V.M.; Jorstad, S.G.; Marscher, Alan P.; Weaver, Z.R.; Acosta-Pulido, J.A.; Agudo, I.; Andreeva, T.; Arkharov, A.; Bachev, R.; Benítez, E.; Berton, M.; Björklund, I.; Borman, G.A.; Bozhilov, V.; Carnerero, M.I.; Carosati, D.; Casadio, C.; Chen, W.P.; Damljanovic, G.; D’Ammando, F.; Escudero, J.; Fuentes, A.; Giroletti, M.; Grishina, T.S.; Gupta, A.C.; Hagen-Thorn, V.A.; Hart, M.; Hiriart, D.; Hou, W.-J.; Ivanov, D.; Kim, J.-Y.; Kimeridze, G.N.; Konstantopoulou, C.; Kopatskaya, E.N.; Kurtanidze, O.M.; Kurtanidze, S.O.; Lähteenmäki, A.; Larionova, E.G.; Larionova, L.V.; Marchili, N.; Markovic, G.; Minev, M.; Morozova, D.A.; Myserlis, I.; Nakamura, M.; Nikiforova, A.A.; Nikolashvili, M.G.; Otero-Santos, J.; Ovcharov, E.; Pursimo, T.; Rahimov, I.; Righini, S.; Sakamoto, T.; Savchenko, S.S.; Semkov, E.H.; Shakhovskoy, D.; Sigua, L.A.; Stojanovic, M.; Strigachev, A.; Thum, C.; Tornikoski, M.; Traianou, E.; Troitskaya, Y.V.; Troitskiy, I.S.; Tsai, A.; Valcheva, A.; Vasilyev, A.A.; Vince, O.; Zaharieva, E.
Among active galactic nuclei, blazars show extreme variability properties. We here investigate the case of the BL Lac object S4 0954+65 with data acquired in 2019–2020 by the Transiting Exoplanet Survey Satellite (TESS) and by the Whole Earth Blazar Telescope (WEBT) Collaboration. The 2-min cadence optical light curves provided by TESS during three observing sectors of nearly 1 month each allow us to study the fast variability in great detail. We identify several characteristic short-term time-scales, ranging from a few hours to a few days. However, these are not persistent, as they differ in the various TESS sectors. The long-term photometric and polarimetric optical and radio monitoring undertaken by the WEBT brings significant additional information, revealing that (i) in the optical, long-term flux changes are almost achromatic, while the short-term ones are strongly chromatic; (ii) the radio flux variations at 37 GHz follow those in the optical with a delay of about 3 weeks; (iii) the range of variation of the polarization degree and angle is much larger in the optical than in the radio band, but the mean polarization angles are similar; (iv) the optical long-term variability is characterized by a quasi-periodicity of about 1 month. We explain the source behaviour in terms of a rotating inhomogeneous helical jet, whose pitch angle can change in time.
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