Multiband flux and spectral variability study of the flaring activity in BL Lacertae during its 2020 outburst

ABSTRACT In this work, we present the results of recent quasi-simultaneous multiband optical observations (in BVRI) of the blazar BL Lacertae over diverse time-scales. For this study, the source was observed from 2020 September–October using six different telescopes around the world, collecting $\sim$5800 photometric image frames in BVRI. The source displays many episodes of significant intraday variability, and the amplitude increases with the brightness of the target. Moreover, the object also showed significant variability on a short-term basis, with flux variability amplitudes of 85.6 per cent, 78.9 per cent, 93.4 per cent, and 67.6 per cent in B, V, R, and I passbands, respectively. The colour–magnitude analysis of the source displays dominantly bluer-when-brighter behaviour on both intraday and short time-scales, which can be attributed to the energetic processes in the jet. Correlation analysis points towards a strong correlation between optical bands with no firmly detectable time lags. Moreover, we also performed a periodicity search using the Lomb–Scargle and weighted wavelet Z-transform methods and found plausible indications of the presence of quasi-periodicity in the blazar. We also generated spectral energy distributions for nights with quasi-simultaneous observations in all four bands and found the spectral index to range from 2.9–3.2, which can be ascribed to a strong jet contribution. We discuss the results within the framework of prospective source intrinsic and extrinsic scenarios.