Context : BL Lac objects are an extreme type of active galactic nuclei ( AGNs ) that belong to the largest population of \gamma -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 the \gamma -rays .
Two strong connections that link radio and mid-infrared emission of blazars to the emission in the \gamma -ray band are well established .
They constitute the basis for associating \gamma -ray sources with their low-energy counterparts .

Aims : We searched for a possible link between X-ray and \gamma -ray emissions for the subclass of BL Lacs using all archival Swift/XRT observations combined with Fermi data for a selected sample of 351 sources .

Methods : Analyzing \sim 2400 ks of Swift/XRT observations that were carried out until December 2018 , we discovered that above the \gamma -ray flux threshold F _ { \gamma } \approx 3 \times 10 ^ { -12 } erg cm ^ { -2 } s ^ { -1 } , 96 % of all Fermi BL Lacs have an X-ray counterpart that is detected with signal-to-noise ratio higher than 3 .

Results : We did not find any correlation or clear trend between X-ray and \gamma -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 .

Conclusions :