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

Abstract

BL Lacertae (BL Lac) categorized as a source of high energy photon in the TeV catalog and considered as a possible source of astrophysical neutrinos. The source has shown the brightest X-ray flare ever detected from it. A detailed study can answer many puzzling questions related to multiband emission and fast 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. A broadband SED modeling is done to answer the possible physical mechanism responsible for broadband emission. We found that the source has crossed all its previous limits of flux and reached to a maximum ever seen from this source in optical and X-rays. In X-ray, the fractional variability is found to be above 100% (1.8397±0.0181) within a time span of a day. The fastest X-ray variability time is estimated as 11.28 hours within a matter of a day, which is seen first time in this source. The other multiband emissions are found to be highly correlated with X-ray within the one day of time lag. The broadband SED modeling requires two different location sites two explain the low and high flux state. A significant spectral change was observed in the optical-UV and X-ray spectrum which eventually leads to the result that the synchrotron peak is shifted towards the higher energy and BL Lac behaves like an HBL type source during the high state.

A thirteen-year-long broadband view of BL Lac

“…During the brightest and softest X-ray state (on MJD 59128.91), the low-energy SED component extended to the X-ray band as a consequence of a significant change of the location of the synchrotron peak from the usual ∼ 10 14 Hz to ∼ 10 16 Hz, well into the HBL regime (Padovani & Giommi 1995;Abdo et al 2010). Such a large modification, never observed before in BL Lac , marks the extraordinary nature of this flare, which has been studied also by D'Ammando (2021a) and Prince (2021).…”

Astrophysical Neutrinos and Blazars

“…IBL blazars also show large ν S peak changes which cause them to occasionally become HBLs during high states. One such example is shown on the left side of Figure 3 for the case of BL Lac, which shifted ν S peak from typical values around 10 14 Hz to ∼ 10 16 Hz during a large flare [59,60]. Several other IBL/HBL transitional sources are known; some of these, e.g., OJ 287, TXS 0506+056, PKS 2005−489, PG 1553+113, are among the blazars that have been frequently observed by Swift, and their SEDs can be found in Ref.…”

Astrophysical Neutrinos and Blazars

“…IBL blazars also show large ν S peak changes which cause them to occasionally become HBLs during high states. One such example is shown on the left side of Figure 3 for the case of BL Lac, which shifted ν S peak from typical values around 10 14 Hz to ∼10 16 Hz during a large flare [59,60]. Several other IBL/HBL transitional sources are known; some of these, e.g., OJ 287, TXS 0506+056, PKS 2005−489, PG 1553+113, are among the blazars that have been frequently observed by Swift, and their SEDs can be found in ref.…”