Abstract:
In 2022 the BL Lac object S4 0954+65 underwent a major variability phase, reaching its historical maximum brightness in the
optical and γ -ray bands. We present optical photometric and polarimetric data acquired by the Whole Earth Blazar Telescope
(WEBT) Collaboration from 2022 April 6 to July 6. Many episodes of unprecedented fast variability were detected, implying
an upper limit to the size of the emitting region as low as 10−4 parsec. The WEBT data show rapid variability in both the degree
and angle of polarization. We analyse different models to explain the polarization behaviour in the framework of a twisting
jet model, which assumes that the long-term trend of the flux is produced by variations in the emitting region viewing angle.
All the models can reproduce the average trend of the polarization degree, and can account for its general anticorrelation with
the flux, but the dispersion of the data requires the presence of intrinsic mechanisms, such as turbulence, shocks, or magnetic
reconnection. The WEBT optical data are compared to γ -ray data from the Fermi satellite. These are analysed with both fixed
and adaptive binning procedures. We show that the strong correlation between optical and γ -ray data without measurable delay
assumes different slopes in faint and high brightness states, and this is compatible with a scenario where in faint states we mainly
see the imprint of the geometrical effects, while in bright states the synchrotron self-Compton process dominates.
