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
  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. 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. Analyzing $\sim$2400 ks of Swift/XRT observations that were
carried out until December 2018, we discovered that above the $\gamma$-ray flux
threshold
$F_{\gamma}\approx3\times10^{-12}\,\rm{erg}\,\rm{cm}^{-2}\,\rm{s}^{-1}$, 96\%
of all \emph{Fermi} BL Lacs have an X-ray counterpart that is detected with
signal-to-noise ratio higher than 3. 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.
