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

Abstract

We have obtained simultaneous and continuous photo-polarization observations of the blazar BL Lacertae in optical and near-infrared (NIR) bands during an historical outburst from 2020 to 2021. In total, 14 nights of observations were performed where 10 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 an 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.

The Microvariability and Wavelength Dependence of Polarization Vector of BL Lacertae in the Outburst 2020 to 2021

“…We note that most of the days showed monotonic rise or decay with amplitudes of ∼0.1 mag in a few hours. The all light curves and polarization degree and angle variations are shown in Imazawa et al (2022). In 2021 July 31, a microvariability of the flux with a timescale of a few minutes with an amplitude 0.01-0.02 mag was found.…”

“…where z is the redshift (z = 0.069, Miller & Hawley 1977) and ΔF is the peak of the flux from which the baseline component is subtracted (ΔF = 4.60 × 10 −12 erg cm −2 s −1 ). As a result of timescale estimation using linear function fitting (details in Imazawa et al (2022)), the rise and decay times are determined to be 340 ± 45 seconds and the decay time is 278 ± 37 seconds for 31 July 2021. We confirmed that the variability is significant because the variability parameter C/Γ = 5.21 after subtracting the baseline for this variability (Romero et al 2002;Cellone et al 2007).…”

“…The duration of the observations ranged from one to five hours per night and were performed over 14 nights during the bright period of BL Lac, from September 2020 to August 2021. The detailed observation and analysis method is based on Imazawa et al (2022).…”

Intraday variations of polarization vector in blazars: a key to the optical jet structure?

“…Our monitoring data is marked with circles. The green vertical bars indicate optical observations from(Imazawa et al 2023), and orange dashed lines are for IXPE epochs from(Middei et al 2023). Right: QU -diagram for the polarization difference between the "redder" and "bluer" bands (details inShablovinskaya et al 2023)…”