Optical flux and spectral variability of BL Lacertae during its historical high outburst in 2020

Abstract

BL Lacertae had undergone a series of historical high flux activity over a year, from August 2020 in the optical to VHE $\gamma$-rays. In this paper, we report on optical flux and spectral variability of the first historical maxima outburst event during October-November in g, r and i bands with the 1.26m telescope at Xinglong observatory, China. We detected significant intranight variations with amplitude rising up to $\sim 30$%, when the fastest variability timescale is found to be a few tens of minutes, giving an emitting region size of the order $10^{-3}$ pc, which corresponds to $\sim 100$ Schwarzschild radius of the central black hole, likely coming from some jet mini-structures. Unlike on intranight timescale, a clear frequency dependent pattern along with symmetric timescales ($\sim$ 11d) of flux variation are detected on 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 clustered in two distinct branches in color--magnitude diagram within a period of $\sim$ 6 hours that is connected to a hard-soft-hard spectral evolution trend extracted from time-resolved spectra. Such trend has never seen in BL Lac or any other blazars before to the best of our knowledge. The results obtained in this study can be explained in the context of shock induced particle acceleration or magnetic re-connection in the jet where turbulent processes most likely resulted the asymmetric flux variation on nightly timescale.

15 pages, 3 tables, 8 figures. Accepted for publication in ApJ

References in corpus (20)

• Fast TeV variability in blazars: jets in a jet
• Relativistic Jets Shine through Shocks or Magnetic Reconnection?
• The Spectral Energy Distributions of Fermi Blazars
• Multi-colour optical monitoring of eight red blazars
• Blazar flares powered by plasmoids in relativistic reconnection
• Stochastic particle acceleration and synchrotron self--Compton radiation in TeV blazars
• Discovery of very high energy gamma-ray emission from the low-frequency peaked BL Lacertae object BL Lacertae
• Multi-band optical variability studies of BL Lacertae
• Longterm Optical Monitoring of Bright BL Lacertae Objects with ATOM: Spectral Variability and Multiwavelength Correlations
• Structure and flux variability in the VLBI jet of BL Lacertae during the WEBT campaigns (1995--2004)
• On the reliability of microvariability tests in quasars
• Studies of the Jet in BL Lacertae. II. Superluminal Alfvén Waves
• Particle-acceleration timescales in TeV blazar flares
• A search for spectral hysteresis and energy-dependent time lags from X-ray and TeV gamma-ray observations of Mrk 421
• A thirteen-year-long broadband view of BL Lac
• On the rapid TeV flaring activity of Markarian 501
• Broadband study of BL Lac during flare of 2020: Spectral evolution and emergence of HBL component
• NICER, NuSTAR, and Swift follow-up observations of the $\gamma$-ray flaring blazar BL Lacertae in 2020 August-October
• Optical variability of a newly discovered blazar sample from the BZCAT Catalog
• Robust Automated Photometry Pipeline for Blurred Images

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