The origin of mid-infrared colors in γ-ray blazars

¹ INFN – Istituto Nazionale di Fisica Nucleare, Sezione di Torino, Via Pietro Giuria 1, I-10125 Turin, Italy
² Dipartimento di Fisica, Università degli Studi di Torino, Via Pietro Giuria 1, I-10125 Torino, Italy
³ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 20138, USA
⁴ INAF-Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
⁵ Consorzio Interuniversitario per la Fisica Spaziale (CIFS), Via Pietro Giuria 1, 10125 Torino, Italy

^⋆ Corresponding author; raniere.m.menezes@gmail.com

Received: 10 October 2024
Accepted: 17 February 2025

Abstract

Context. The combination between non-thermal and thermal emission in γ-ray blazars pushes them to a specific region of the mid-infrared three-dimensional color diagram, the so-called blazar locus, built based on observations performed with the Wide-field Infrared Survey Explorer. The selection of blazar candidates based on these mid-infrared colors has been extensively used in the past decade in the hunt for the counterparts of unassociated γ-ray sources observed with the Fermi Large Area Telescope and in the search for new blazars in optical spectroscopic campaigns.

Aims. In this work, we provide a theoretical description of the origin of the blazar locus and show how we can reasonably reproduce it with a model consisting of only three spectral components: a log-parabola accounting for the non-thermal emission, and an elliptical host and dust torus accounting for the thermal emission.

Methods. We simulate spectral energy distributions (SEDs) for blazars, starting with a pure log-parabola model and then increasing its complexity by adding a template elliptical galaxy and dust torus. From these simulations, we compute the mid-infrared magnitudes and corresponding colors to create our own version of the blazar locus.

Results. Our modeling allows for the selection of spectral parameters that better characterize the mid-infrared emission of γ-ray blazars, such as the log-parabola curvature (β < 0.04 for 50% of our sample) and an average spectral peak around E_p ≈ 1.5 × 10⁻¹³ erg. We also find that the log-parabola is the main spectral component behind the observed mid-infrared blazar colors, although additional components such as a host galaxy and a dust torus are crucial to obtain a precise reconstruction of the blazar locus.