Chromatic optical polarization of BL Lac: while faint and bright

Abstract

Due to the first results on astrophysical X-ray polarization provided by 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.

Analysis of the Intranight Variability of BL Lacertae during Its 2020 August Flare

“…The comparison reveals a good correlation between the optical and γ-ray LCs. In general, the LCs could be split visually into two parts-a preflare and a flare (see also Shablovinskaya et al 2023).…”

“…In a recent study, Kalita et al (2023) reported results from the BL Lacertae monitoring from 2020 October 1 to November 23 in the optical. According to their Table 2, the source showed INV during 4 nights out of 10 (the probably variable cases considered nonvariable); see also Shablovinskaya et al (2023) regarding the source monitoring in that period. Therefore, the duty cycle could be estimated as ∼40%, which is significantly lower than ours.…”

“…We have only B versus I time lag for August 26, and so we can apply the following expression to derive the magnetic field strength (e.g., Tavecchio et al 1998;Papadakis et al 2003):   if we assumed a Doppler factor of 11.0 0.3 0.3 -+  as estimated above (see also Shablovinskaya et al 2023). The uncertainties of  were derived using the lag and Doppler factor randomization.…”

“…It is classified as a low-synchrotron-peaked blazar (Nilsson et al 2018). BL Lacertae has been of great interest for numerous intense multiwavelength (MWL) campaigns (e.g., Villata et al 2002Villata et al , 2003Böttcher et al 2003;Raiteri et al 2010;Wierzcholska et al 2015;Agarwal et al 2017;MAGIC Collaboration et al 2019;Weaver et al 2020;Jorstad et al 2022;Kalita et al 2023;Shablovinskaya et al 2023); in particular, BL Lacertae is one of the favorite targets of the campaigns organized by the Whole Earth Blazar Telescope collaboration. More-than-a-century-long observations of BL Lacertae reveal intense variability on diverse timescales ranging from a few minutes (e.g., Villata et al 2002;Gaur et al 2015;Meng et al 2017;Fang et al 2022) to years (Carini et al 1992;Villata et al 2004aVillata et al , 2004bVillata et al , 2009Raiteri et al 2013).…”

Analysis of the intra-night variability of BL Lacertae during its August 2020 flare

“…BL Lacertae has been of great interest for numerous intense multiwavelength (MWL) campaigns (e.g. Villata et al 2002Villata et al , 2003Böttcher et al 2003;Raiteri et al 2010;Wierzcholska et al 2015;Agarwal et al 2017;MAGIC Collaboration et al 2019;Weaver et al 2020;Jorstad et al 2022;Kalita et al 2023;Shablovinskaya et al 2023); in particular, BL Lacertae is one of the favorite targets of the campaigns organized by the Whole Earth Blazar Telescope collaboration. More than century-long observations of BL Lacertae reveal intense variability on diverse timescales ranging from a few minutes (e.g.…”