Chromatic optical polarization of BL Lac: while faint and bright

TL;DR

This study investigates the wavelength-dependent optical polarization variability of the blazar BL Lac, revealing intraday changes and chromatic behavior that challenge existing shock-in-jet models.

Contribution

It provides detailed multi-band optical polarization observations of BL Lac, highlighting rapid variability and chromatic effects not fully explained by current models.

Findings

Polarization varies intraday with no stable period.

Maximum polarization occurs during minimum brightness.

Chromatism changes during flares, challenging existing models.

Abstract

Due to the first results on astrophysical X-ray polarization provided by \textit{IXPE} observatory, the interest in wavelength-dependent synchrotron polarization of BL Lac type objects increases. This paper presents the results of multi-band optical observations of the well-known blazar named BL Lac ($z=0.069$) in polarized light. It was shown that the object's emission, regardless of its phase of activity, is characterized by the intraday variability of brightness and polarization with changes occurring on a time-scale of up to 1.5 hours without any stable oscillation period. Polarimetric observations in the different optical bands show that the degree and angle of polarization of the blazar depend on the wavelength, and the maximum chromatism, as well as the maximum observed polarization degree, was detected during the minimum brightness state; during the flare state, the polarization chromatism changed along with the flux gradient on the time-scale of an hour. Qualitatively, such behaviour can be described by the shock-in-jet model, yet the chromatism amplitude and its rapid changes differ significantly from the model predictions and challenge the numerical calculations.