A 13-yr-long broad-band view of BL Lac

ABSTRACT We present the results of an extensive analysis optical, ultraviolet, X-ray, and γ-ray data collected from observations BL Lac objects prototype Lacertae carried out over a period nearly 13 yr, between 2008 August 2021 March. The source is characterized by strongly variable emission at all frequencies, often accompanied spectral changes. In band several prominent flares have been detected, largest one reaching flux Fγ(> 196.7 MeV) = (4.39 ± 1.01) × 10−6 photon cm−2 s−1. X-ray variability during brightest flare on MJD 59128.18 (2020 October 6) was softer-when-brighter trend due to shift synchrotron peak ∼1016 Hz, well into HBL domain. widely changing multiwavelength systematically investigated fitting leptonic models that include self-Compton external Compton components 511 high-quality quasi-simultaneous broad-band energy distributions (SEDs). majority selected SEDs can be adequately fitted within one-zone model with reasonable parameters. Only 46 soft bright spectra when observed in very high bands explained two-zone scenario. behaviour interpreted as emergence freshly accelerated particles second zone located beyond broad-line region.