Abstract:
In this work, we report the presence of rapid intra-night optical variations in both flux and polarization of the blazar BL Lacertae
during its unprecedented 2020–2021 high state of brightness. The object showed significant flux variability and some colour
changes, but no firmly detectable time delays between the optical bands. The linear polarization was also highly variable in both
polarization degree and angle (electric vector polarization angle). The object was observed from several observatories throughout
the world, covering a total of almost 300 h during 66 nights. Based on our results, we suggest that the changing Doppler factor of
an ensemble of independent emitting regions, travelling along a curved jet that at some point happens to be closely aligned with
the line of sight, can successfully reproduce our observations during this outburst. This is one of the most extensive variability
studies of the optical polarization of a blazar on intra-night time-scales.
