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

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