The physical properties of Fermi BL Lac objects jets

We investigate the physical properties of BL Lac objects (BL Lacs) jets by modelling the quasi-simultaneous spectral energy distributions (SEDs) of 22 Fermi BL Lacs in the frame of a simple one-zone synchrotron self-Compton (SSC) model. We obtained the best-fitting model parameters and their uncertainties for each BL Lac through the χ²-minimization procedure and discussed their implications on the physical processes. The modelling results show that the one-zone SSC model can successfully fit the SEDs of high-synchrotron-peaked BL Lacs (HBLs) and intermediate-synchrotron-peaked BL Lacs (IBLs), but fails to explain the SEDs of low-synchrotron-peaked BL Lacs. The statistical analysis results for model parameters are summarized as follows. (1) No correlation is found between magnetic field (B) and the broken energy of relativistic electrons distribution ($\gamma ^{\prime }_{\rm b}$) for HBLs and IBLs, but there are inverse correlations between $\gamma ^{\prime }_{\rm b}$ and the radius of emitting blob ($R^{\prime }_{\rm b}$) as well as the electrons number $K^{\prime }_{\rm e}$ for HBLs and IBLs. It is therefore concluded that the variation of $\gamma ^{\prime }_{\rm b}$ is mainly caused by that of $R^{\prime }_{\rm b}$ rather than B for HBLs and IBLs. (2) The Poynting flux in jets cannot account for the observed radiations since the power in Poynting flux is smaller than the radiative power, and the cold protons could be the primary energy carrier in the jets.