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BL Lac objects are an extreme type of active galactic nuclei (AGNs) thatbelong to the largest population of $\gamma$-ray sources: blazars. This classof AGNs shows a double-bumped spectral energy distribution that is commonlydescribed in terms of a synchrotron self-Compton (SSC) emission process,whereas the low-energy component that dominates their emission between theinfrared and the X-ray band is tightly connected to the high-energy componentthat peaks in the $\gamma$-rays. Two strong connections that link radio andmid-infrared emission of blazars to the emission in the $\gamma$-ray band arewell established. They constitute the basis for associating $\gamma$-raysources with their low-energy counterparts. We searched for a possible linkbetween X-ray and $\gamma$-ray emissions for the subclass of BL Lacs using allarchival Swift/XRT observations combined with Fermi data for a selected sampleof 351 sources. Analyzing $\sim$2400 ks of Swift/XRT observations that werecarried out until December 2018, we discovered that above the $\gamma$-ray fluxthreshold$F_{\gamma}\approx3\times10^{-12}\,\rm{erg}\,\rm{cm}^{-2}\,\rm{s}^{-1}$, 96\%of all \emph{Fermi} BL Lacs have an X-ray counterpart that is detected withsignal-to-noise ratio higher than 3. We did not find any correlation or cleartrend between X-ray and $\gamma$-ray fluxes and/or spectral shapes, but wediscovered a correlation between the X-ray flux and the mid-infrared color.Finally, we discuss on a possible interpretation of our results in the SSCframework.

The $\gamma$-ray sky seen at X-ray energies I. Searching for the connection between X-rays and $\gamma$-rays in Fermi BL Lac objects