Shablovinskaya E.S., Malygin E.A., Oparin D.V. (SAO RAS)

    Considering the first results of X-ray polarimetry of blazars and extensive discussion on physical processes in relativistic jets of active nuclei, interest in multi-wavelength polarimetry of objects like BL Lac has now increased. This paper presents the results of multi-band optical observations of the well-known blazar BL Lac (z = 0.069) in polarized light obtained with the 1-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences in 2020–2022. It is shown that the emission of the object, regardless of the phase of its activity, is characterized by intraday variability of brightness and polarization with changes occurring on a time-scale of up to 1.5 hours without any designated period (Fig. 1). It is important that the polarization color shows variability on an hour scale. The observation results confirm that the object BL Laceratae exhibits the anti-correlation between brightness and degree of polarization that is qualitatively described by the shock-wave model in a turbulent jet plasma. For the first time, the dependence of the polarization color on the phase of the object activity was discovered (Fig. 2), which indicates the dominance of different physical processes in the plasma and, together with the rapid variability of the polarization color, challenges modern numerical calculations.

Fig. 1. The intraday variability of the polarization vector of BL Lac during the night in the V and I bands. Numbers 1 and 2 indicate sharp changes in the direction of the vector corresponding to the change in the flux derivative; a question mark indicates the change in the direction of the vector not accompanied by a change in the light curve.

Fig. 2. Dependence of the polarization chromatism on the observation epoch.

The work was carried out within the framework of the state assignment of the Special Astrophysical Observatory of the Russian Academy of Sciences approved by the Ministry of Science and Higher Education of the Russian Federation.

Published:
Shablovinskaya E., Malygin E., Oparin D.; Chromatic optical polarization of BL Lac: while faint and bright, MNRAS, 519, 3, 3798-3810, 10.1093/mnras/stac3775 (2023).