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

TL;DR

This study analyzes intra-night optical variability of BL Lacertae during its August 2020 flare, revealing chromatic behavior, inter-band time lags, and characteristics of the emission region using multi-telescope data over 84 nights.

Contribution

It provides a detailed analysis of intra-night variability during a flare, including flare decomposition and physical parameter estimation within the turbulent jet model.

Findings

BL Lacertae showed ~2.2 mag flux variation in R band.

Predominantly bluer-when-brighter chromatism observed.

Detected inter-band time lags of a few minutes.

Abstract

We present an analysis of the $BVRI$ photometry of the blazar BL Lacertae on diverse timescales from mid-July to mid-September 2020. We have used 11 different optical telescopes around the world and have collected data over 84 observational nights. The observations cover the onset of a new activity phase of BL Lacertae started in August 2020 (termed as the August 2020 flare by us), and the analysis is focused on the intra-night variability. On short-term timescales, (i) flux varied with ~2.2\,mag in $R$ band, (ii) the spectral index was found to be weakly dependent on the flux (i.e., the variations could be considered mildly chromatic) and (iii) no periodicity was detected. On intra-night timescales, BL Lacertae was found to show bluer-when-brighter chromatism predominantly. We also found two cases of significant inter-band time lags of the order of a few minutes. The duty cycle of the blazar during the August 2020 flare was estimated to be quite high (~90\% or higher). We decomposed the intra-night light curves into individual flares and determined their characteristics. On the basis of our analysis and assuming the turbulent jet model, we determined some characteristics of the emitting regions: Doppler factor, magnetic field strength, electron Lorentz factor, and radius. The radii determined were discussed in the framework of the Kolmogorov theory of turbulence. We also estimated the weighted mean structure function slope on intra-night timescales, related it to the slope of the power spectral density, and discussed it with regard to the origin of intra-night variability.