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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 $F_{gamma}approx3times10^{-12},rm{erg},rm{cm}^{-2},rm{s}^{-1}$, 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.
We present the spectral signatures of the Bethe-Heitler pair production ($pe$) process on the spectral energy distribution (SED) of blazars, in scenarios where the hard $gamma$-ray emission is of photohadronic origin. If relativistic protons interact
Blazars represent the most abundant class of high-energy extragalactic $gamma$-ray sources. The subset of blazars known as BL Lac objects is on average closer to Earth and characterized by harder spectra at high energy than the whole sample. The frac
The complex interplay of processes at the Galactic Center is at the heart of numerous past, present, and (likely) future mysteries. We aim at a more complete understanding of how spectra extending to >10 TeV result. We first construct a simplified mo
Peaking at 3.7 mag on 2020 July 11, YZ Ret was the second-brightest nova of the decade. The novas moderate proximity (2.7 kpc from Gaia) provided an opportunity to explore its multi-wavelength properties in great detail. Here we report on YZ Ret as p
In this paper, we compile the very-high-energy and high-energy spectral indices of 43 BL Lac objects from the literature. Based on a simple math model, $DeltaGamma_{obs}=alpha {rm{z}}+beta $, we present evidence for the origin of an observed spectral