Intra-night optical flux and polarization variability of BL Lacertae during its 2020-2021 high state

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.

Automated summary

In this study, we observed rapid intra-night optical variations in flux and polarization of the blazar BL Lacertae during its high state of brightness. The object exhibited significant flux variability and some color changes without detectable time delays between optical bands. The linear polarization also showed high variability in both polarization degree and angle. Our observations suggest that changing Doppler factor in independent emitting regions traveling along a curved jet closely aligned with the line of sight can explain these variations during the outburst. This study provides one of the most extensive investigations of the intra-night variability in the optical polarization of a blazar.