Broadband study of BL Lac during flare of 2020: Spectral evolution and emergence of HBL component

TL;DR

This study analyzes the 2020 flare of BL Lac, revealing extreme variability, spectral evolution, and the emergence of a new high-energy component, advancing understanding of blazar emission mechanisms.

Contribution

It provides the first detailed temporal and spectral analysis of the brightest flare, showing the emergence of an HBL component and requiring multi-region modeling.

Findings

BL Lac reached its maximum flux in optical and X-rays.

X-ray variability amplitude exceeded 100%.

Spectral shifts indicate emergence of a new HBL component.

Abstract

BL Lacertae (BL Lac) is categorized as TeV blazar and considered as a possible source of astrophysical neutrinos. In 2020, the brightest X-ray flare ever detected from it. A detailed study can answer many puzzling questions related to multiband emissions and fast-flux variability often seen in this kind of source. We have performed the temporal and spectral analysis of the brightest flare. The variability is characterized by the fractional variability amplitude and the variability time. We found that the source has crossed all its previous limits of flux and reached the maximum ever seen from it in optical and X-rays. It is highly variable in X-rays with fractional variability above 100$\%$ (1.8397$\pm$0.0181) and the fastest variability time of 11.28 hours within a day. The broadband light curves correlation with X-ray suggest a time lag of one day. A broadband SED modeling is pursued to understand the possible physical mechanisms responsible for broadband emission. Modeling requires two emission regions located at two different sites to explain the low and high flux states. A significant spectral change is observed in the optical-UV and X-ray spectrum during the high state, which eventually leads to shifts in the location of the synchrotron peak towards higher energy, suggesting an emergence of a new HBL component.