Abstract:
In 2021 BL Lacertae underwent an extraordinary activity phase, which was intensively followed by the Whole Earth Blazar
Telescope (WEBT) Collaboration. We present the WEBT optical data in the BVRI bands acquired at 36 observatories around
the world. In mid-2021 the source showed its historical maximum, with R = 11.14. The light curves display many episodes of
intraday variability, whose amplitude increases with source brightness, in agreement with a geometrical interpretation of the
long-term flux behaviour. This is also supported by the long-term spectral variability, with an almost achromatic trend with
brightness. In contrast, short-term variations are found to be strongly chromatic and are ascribed to energetic processes in the jet.
We also analyse the optical polarimetric behaviour, finding evidence of a strong correlation between the intrinsic fast variations
in flux density and those in polarization degree, with a time delay of about 13 h. This suggests a common physical origin. The
overall behaviour of the source can be interpreted as the result of two mechanisms: variability on time-scales greater than several
days is likely produced by orientation effects, while either shock waves propagating in the jet, or magnetic reconnection, possibly
induced by kink instabilities in the jet, can explain variability on shorter time-scales. The latter scenario could also account for
the appearance of quasi-periodic oscillations, with periods from a few days to a few hours, during outbursts, when the jet is more
closely aligned with our line of sight and the time-scales are shortened by relativistic effects.
