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Optical Flux and Spectral Variability of BL Lacertae during Its Historical High Outburst in 2020

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dc.contributor.author Kalita, Nibedita, et.al.
dc.contributor.author Gupta, Alok C.
dc.date.accessioned 2024-03-11T05:51:03Z
dc.date.available 2024-03-11T05:51:03Z
dc.date.issued 2023-02
dc.identifier.uri https://doi.org/10.3847/1538-4357/aca801
dc.identifier.uri http://localhost:8080/xmlui/handle/123456789/1511
dc.description.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. en_US
dc.language.iso en en_US
dc.publisher The Astrophysical Journal en_US
dc.relation.ispartofseries 1953;apj943-135
dc.subject Active galactic nuclei en_US
dc.subject Blazars en_US
dc.title Optical Flux and Spectral Variability of BL Lacertae during Its Historical High Outburst in 2020 en_US
dc.type Article en_US


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