Properties and Possible Physical Origins of $γ$-ray Emission in Extreme Synchrotron Blazars

Abstract: Extreme synchrotron blazars, characterized by a first peak in their broadband spectral energy distributions (SEDs) at frequencies exceeding $10^{17}$ Hz, often exhibit a second peak beyond 1~TeV. These sources serve as ideal laboratories for studying particle acceleration and radiation mechanisms in relativistic jets. In this work, we systematically analyze the $\sim$16-year Fermi-LAT observational data for 25 extreme high-synchrotron-peaked BL Lacs (EHBLs). The results indicate that the majority of these sources display stable or low flux levels in the GeV band, with only 6 sources showing significant variability at a confidence level exceeding 5$σ$. The time-averaged spectra over the 16-year period for most EHBLs are well described by a hard power-law model, with photon indices predominantly clustered between 1.7 and 1.8. Using Fermi-LAT data in conjunction with multiwavelength observations compiled from the literature, we construct broadband SEDs for these EHBLs and fit them with a one-zone synchrotron + synchrotron-self-Compton (SSC) model. We find that this simplified theoretical framework is sufficient for modeling the observed SEDs of most of these EHBLs, albeit requiring relatively higher electron energies compared to other $γ$-ray emitting HBLs, and at times under-representing the UV emission. Based on the SED fitting results, we investigate the physical properties of the emission regions in these EHBLs and compare them with those of other $γ$-ray emitting HBLs. Consistent with other GeV--TeV $γ$-ray-emitting BL Lacs, the jets in these EHBLs are marked by low radiation efficiency and low magnetization.