The complex variability of blazars: Time-scales and periodicity analysis
  in S4 0954+65
  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 one 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 three 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 one month. We explain the source behaviour in terms
of a rotating inhomogeneous helical jet, whose pitch angle can change in time.
