A thirteen-year-long broadband view of BL Lac
  We present the results of an extensive analysis of the optical, ultraviolet,
X-ray and $\gamma$-ray data collected from the observations of the BL Lac
objects prototype BL Lacertae carried out over a period of nearly 13 years,
between August 2008 and March 2021. The source is characterized by strongly
variable emission at all frequencies, often accompanied by spectral changes. In
the $\gamma$-ray band several prominent flares have been detected, the largest
one reaching the flux of $F_{\rm \gamma}(>196.7\: {\rm
MeV})=(4.39\pm1.01)\times10^{-6}\:{\rm photon\:cm^{-2}\:s^{-1}}$. The X-ray
spectral variability of the source during the brightest flare on MJD 59128.18
(06 October 2020) was characterized by a softer-when-brighter trend due to a
shift of the synchrotron peak to $\sim 10^{16}$ 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 $\gamma$-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.
