Exceptional X-ray activity in BL Lacertae

BL Lacertae is a unique blazar for which the X-ray band can cover either the synchrotron or the inverse Compton, or both parts of the broadband spectral energy distribution. In the latter case, when the spectral upturn is located in the X-ray range, it allows contemporaneous study of the low- and high-energy ends of the electron distribution function. In this work, we study spectral and temporal variability using X-ray and optical observations of the blazar performed with the Neil Gehrels Swift Observatory from 2020 to 2023. The large set of observational data reveals intensive flaring activity, accompanied by spectral changes in both spectral branches. We conclude that the low-energy and high-energy ends of the particle distribution function are characterised by similar variability scales. Additionally, the hard X-ray observations of BL Lacertae performed with the Nuclear Spectroscopic Telescope Array (NuSTAR) confirm a concave spectral curvature for some epochs of the blazar activity and reveal that it can be shifted up to energies of as high as 8 keV. The time-resolved spectral analysis allows us to disentangle X-ray spectral variability features of the synchrotron from inverse Compton components. Despite significant variability of both spectral components, we find only small changes in the position of the spectral upturn. The different slopes and shapes of the X-ray spectrum of BL Lacertae demonstrate that the classification of this source is not constant, and BL Lacertae can exhibit features of either high-, intermediate-, or low-energy peaked blazar in different epochs of observation. This also indicates that the spectral upturn for this blazar can be located not only in the X-ray range of 0.3-10 keV, but also at lower or higher energies.