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. (arXiv:2004.02968v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Marchesini_E/0/1/0/all/0/1">E. J. Marchesini</a> (UniTO, UniLP, INFN-To, CONICET-UNLP, INAF-OATo), <a href="http://arxiv.org/find/astro-ph/1/au:+Paggi_A/0/1/0/all/0/1">A. Paggi</a> (UniTO), <a href="http://arxiv.org/find/astro-ph/1/au:+Massaro_F/0/1/0/all/0/1">F. Massaro</a> (UniTO), <a href="http://arxiv.org/find/astro-ph/1/au:+Masetti_N/0/1/0/all/0/1">N. Masetti</a> (INAF-OAS), <a href="http://arxiv.org/find/astro-ph/1/au:+DAbrusco_R/0/1/0/all/0/1">R. D'Abrusco</a> (SAO), <a href="http://arxiv.org/find/astro-ph/1/au:+Andruchow_I/0/1/0/all/0/1">I. Andruchow</a> (UniLP, CONICET-UniLP), <a href="http://arxiv.org/find/astro-ph/1/au:+Menezes_R/0/1/0/all/0/1">R. de Menezes</a> (UniTO, Universidade de Sao Paulo)
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.
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.
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