The variability properties of extreme blazars: insights from high-synchrotron-peaked BL Lacs

ABSTRACT We have investigated the spectral and variability behaviours of a distinct sample of 16 high-synchrotron-peaked BL Lac objects (HBLs) on long time-scales, utilizing nearly 16 yr of observations from Fermi-LAT, Swift X-Ray Telescope, and Swift-Ultraviolet and Optical Telescope. Based on long-period $\gamma$-ray and X-ray light curves analysis, we processed the quasi-simultaneous multiwavelength observations of the sample in both low- and high-flux states and classified the HBLs according to their spectral index and flux variations. We then adopt a one-zone leptonic model incorporating both isotropic and anisotropic electron distributions to reproduce the quasi-simultaneous spectral energy distributions (SEDs) of the sample across various activity states, thereby probing the underlying radiation mechanisms. Our main results can be summarized as follows. (1) The X-ray and $\gamma$-ray spectra of most HBLs with high variability exhibit an hardening of the spectrum in the high-flux state, accompanied by a shift in the SED peak frequency. (2) Compared to the traditional homogeneous SSC model, the anisotropic SSC model can yield smaller Doppler factors and stronger magnetic fields. (3) HBLs satisfy the relation ${P_{\rm {e}}}$ (the kinetic power carried in relativistic electrons) $\sim$ ${P_{{B^\prime }}}$ (Poynting flux) $\le$ ${P_{{\rm {rad}}}}$ (the radiative power) $<$ ${P_{\rm {p}}}$ (the kinetic power in cold protons) in different activity states in the anisotropic one-zone SSC model.