Abstract:
BL Lacertae underwent a series of historical high flux activity over a year from 2020 August in the optical to VHE
γ-rays. In this paper, we report on optical flux and spectral variability of the first historical maxima outburst event
during October–November in the g, r, and i bands with the 1.26 m telescope at the Xinglong Observatory, China.
We detected significant intranight variations with amplitude rising up to ∼30%, where the fastest variability
timescale was found to be a few tens of minutes, giving an emitting region size of the order 10−3 pc, which
corresponds to ∼100 Schwarzschild radius of the central black hole, likely coming from some jet mini-structures.
Unlike on the intranight timescale, a clear frequency-dependent pattern along symmetric timescales (∼11 days) of
flux variation is detected on a long timescale. The spectral evolution was predominated by flattening of the spectra
with increasing brightness i.e., a bluer-when-brighter trend in 96% of the cases. On the night before the outburst
peak, the color indices shown in a color–magnitude diagram, clustered into two distinct branches, within a period
of ∼6 hr, which is connected to a hard-soft-hard spectral evolution trend extracted from time-resolved spectra. To
the best of our knowledge, such a trend has never been seen in BL Lac or any other blazars before. The results
obtained in this study can be explained in the context of shock-induced particle acceleration or magnetic
reconnection in the jet where turbulent processes most likely resulted in the asymmetric flux variation on a nightly
timescale.
