The microvariability and wavelength dependence of polarization degree/angle of BL Lacertae in the outburst 2020 to 2021

TL;DR

This study presents simultaneous optical and NIR polarization observations of BL Lacertae during its 2020-2021 outburst, revealing microvariability, wavelength-dependent polarization, and complex emission regions challenging simple jet models.

Contribution

First simultaneous optical and NIR polarization monitoring of BL Lacertae during an outburst, highlighting complex emission regions and challenging existing jet models.

Findings

Microvariability observed on timescales of minutes to hours.

Significant wavelength-dependent polarization degree and angle.

Evidence for multiple emission regions beyond simple models.

Abstract

We have obtained simultaneous and continuous photo-polarization observations of the blazar BL Lacertae in optical and near-infrared (NIR) bands during a historical outburst from 2020 to 2021. In total, fourteen nights of observations were performed where ten observations show microvariability on timescales of a few minutes to several hours. This suggests a compact emission region, and the timescales are difficult to explain by a one-zone shock-in-jet model. Moreover, we found significant differences in the polarization degree (PD) and angle between optical and NIR bands. Nine nights showed a PD in the optical band that is greater than or equal to that in the NIR band, which can be explained by either a shock-in-jet model or the Turbulent Extreme Multi-Zone (TEMZ) model. On the other hand, five nights showed higher PD in a NIR band than an optical band, which cannot be explained by simple shock-in-jet models nor the simple TEMZ model. The observed timescales and wavelength dependency of the PD and polarization angle suggest the existence of complicated multiple emission regions such as an irregular TEMZ model.