Volume 631, November 2019
|Number of page(s)||12|
|Published online||13 November 2019|
The γ-ray sky seen at X-ray energies
I. Searching for the connection between X-rays and γ-rays in Fermi BL Lac objects⋆
Dipartimento di Fisica, Università degli Studi di Torino, Via Pietro Giuria 1, 10125 Turin, Italy
2 Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque, B1900FWA La Plata, Argentina
3 INFN – Istituto Nazionale di Fisica Nucleare, Sezione di Torino, Via Pietro Giuria 1, 10125 Turin, Italy
4 Instituto de Astrofísica de La Plata, CONICET–UNLP, CCT La Plata, Paseo del Bosque, B1900FWA La Plata, Argentina
5 INAF – Osservatorio di Astrofisica e Scienza dello Spazio, Via Gobetti 93/3, 40129 Bologna, Italy
6 Departamento de Ciencias Fisicas, Universidad Andres Bello, Fernandez Concha 700, Las Condes, Santiago, Chile
7 Center for Astrophysics | Harvard & Smithsonian, 60 Garden St, Cambridge, MA 02138, USA
8 Universidade de São Paulo, Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Departamento de Astronomia, São Paulo, SP 05508-090, Brazil
Accepted: 3 September 2019
Context. BL Lac objects are an extreme type of active galactic nuclei (AGNs) that belong to the largest population of γ-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 γ-rays. Two strong connections that link radio and mid-infrared emission of blazars to the emission in the γ-ray band are well established. They constitute the basis for associating γ-ray sources with their low-energy counterparts.
Aims. We searched for a possible link between X-ray and γ-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 ∼2400 ks of Swift/XRT observations that were carried out until December 2018, we discovered that above the γ-ray flux threshold Fγ ≈ 3 × 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 > 3.
Results. We did not find any correlation or clear trend between X-ray and γ-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.
Key words: galaxies: active / galaxies: nuclei / galaxies: jets / BL Lacertae objects: general / X-rays: galaxies / gamma rays: galaxies
Full Table 1 is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (220.127.116.11) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/631/A150
© ESO 2019
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