The complex variability of blazars: Time-scales and periodicity analysis in S4 0954+65

C. M. Raiteri*, M. Villata, V. M. Larionov, S. G. Jorstad, A. P. Marscher, Z. R. Weaver, J. A. Acosta-Pulido, I. Agudo, T. Andreeva, A. Arkharov, R. Bachev, E. Benítez, M. Berton, I. Björklund, G. A. Borman, V. Bozhilov, M. I. Carnerero, D. Carosati, C. Casadio, W. P. ChenG. Damljanovic, F. D'Ammando, J. Escudero, A. Fuentes, M. Giroletti, T. S. Grishina, A. C. Gupta, V. A. Hagen-Thorn, M. Hart, D. Hiriart, W. J. Hou, D. Ivanov, J. Y. Kim, G. N. Kimeridze, C. Konstantopoulou, E. N. Kopatskaya, O. M. Kurtanidze, S. O. Kurtanidze, A. Lähteenmäki, E. G. Larionova, L. V. Larionova, N. Marchili, G. Markovic, M. Minev, D. A. Morozova, I. Myserlis, M. Nakamura, A. A. Nikiforova, M. G. Nikolashvili, J. Otero-Santos, E. Ovcharov, T. Pursimo, I. Rahimov, S. Righini, T. Sakamoto, S. S. Savchenko, E. H. Semkov, D. Shakhovskoy, L. A. Sigua, M. Stojanovic, A. Strigachev, C. Thum, M. Tornikoski, E. Traianou, Y. V. Troitskaya, I. S. Troitskiy, A. Tsai, A. Valcheva, A. A. Vasilyev, O. Vince, E. Zaharieva

*Corresponding author for this work

Research output: Contribution to journalArticleScientificpeer-review

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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.

Original languageEnglish
Pages (from-to)5629-5646
Number of pages18
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
Publication statusPublished - 1 Jul 2021
MoE publication typeA1 Journal article-refereed


  • BL Lacertae objects: general
  • BL Lacertae objects: individual: S4 0954+65
  • Galaxies: active
  • Galaxies: jets


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