摘要：Context.BL Lac objects are an extreme type of active galactic nuclei (AGNs) that belong to the largest population ofγ-ray sources: blazars. This class of AGNs shows a double-bumped spectral energy distribution that is commonly described in terms of a synchrotron self-Compton (SSC) emission process, whereas the low-energy component that dominates their emission between the infrared and the X-ray band is tightly connected to the high-energy component that peaks in theγ-rays. Two strong connections that link radio and mid-infrared emission of blazars to the emission in theγ-ray band are well established. They constitute the basis for associatingγ-ray sources with their low-energy counterparts.Aims.We searched for a possible link between X-ray andγ-ray emissions for the subclass of BL Lacs using all archivalSwift/XRT observations combined withFermidata for a selected sample of 351 sources.Methods.Analyzing ∼2400 ks ofSwift/XRT observations that were carried out until December 2018, we discovered that above theγ-ray flux thresholdFγ ≈ 3 × 10−12erg cm−2s−1, 96% of allFermiBL Lacs have an X-ray counterpart that is detected with signal-to-noise ratio > 3.Results.We did not find any correlation or clear trend between X-ray andγ-ray fluxes and/or spectral shapes, but we discovered a correlation between the X-ray flux and the mid-infrared color. Finally, we discuss on a possible interpretation of our results in the SSC framework.
