Flux and Spectral Variability of High-Energy-Peaked BL Lacertae Objects in the 0.3–10 KeV Band

BL Lac objects are active galactic nuclei notable for beamed nonthermal radiation, which is generated in one of the relativistic jets forming a small angle to our line-of-sight. The broadband spectra of BL Lacs show a two-component spectral energy distribution (SED). High-energy-peaked BL Lacs (HBLs) exhibit their lower-energy (synchrotron) peaks at UV to X-ray frequencies. Consequently, these objects are generally bright in the 0.3-10 keV bands (compared to other blazar subclasses) and allow us to carry out intense timing and spectral studies on the wide range of timescales (from years down to a few minutes). Although x-ray emission of HBLs is widely accepted to have a synchrotron origin, many problems associated with the jet particle content, their acceleration up to ultrarelativistic energies, and unstable mechanisms responsible for the extreme flux and spectral variability still remain to be solved. This review highlights the basic timing and spectral results obtained in the framework of the numerous timing and spectral studies of HBLs in the 0.3-10 keV band which is covered by the X-ray instruments operating onboard the different space missions. Moreover, the plausible physical processes ot be responsible for the observed HBL features (relativistic shocks, magnetic reconnection, turbulence etc.) are also addressed.