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^⋆

¹ Dipartimento di Fisica, Università degli Studi di Torino, Via Pietro Giuria 1, 10125 Turin, Italy
e-mail: ejmarchesini@gmail.com
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

Received: 10 July 2019
Accepted: 3 September 2019

Abstract

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 F_γ ≈ 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.