The optical behaviour of BL Lacertae at its maximum brightness levels: a blend of geometry and energetics

TL;DR

This study analyzes the 2021 maximum brightness activity of BL Lacertae, revealing how geometrical and energetic processes in its jet influence optical variability and polarization, with implications for understanding blazar jet physics.

Contribution

It provides a comprehensive multi-band optical and polarimetric analysis of BL Lacertae during its maximum brightness, highlighting the roles of geometry and energetic jet processes in variability.

Findings

Intraday variability amplitude increases with brightness.

Long-term spectral variability is almost achromatic.

Short-term variations are strongly chromatic and linked to jet energetics.

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 polarisation 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.