SIMBAD references

Extreme synchrotron blazars, characterized by a first peak in their broadband spectral energy distributions (SEDs) at frequencies exceeding 10¹⁷ 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 ∼16 yr Fermi-LAT observational data for 25 extreme high-synchrotron-peaked BL Lacertae (EHBL) objects. The results indicate that the majority of these sources display stable or low flux levels in the GeV band, with only six sources showing significant variability at a confidence level exceeding 5σ. The time-averaged spectra over the 16 yr 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 and 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 Lacertae objects, the jets in these EHBLs are marked by low radiation efficiency and low magnetization.