Linking Fermi blazars and radio galaxies through accretion and jet radiation mechanisms

¹ Dipartimento di Fisica e Astronomia “G. Galilei”, Universitá di Padova, Via F. Marzolo, 8, I-35131 Padova, Italy
² Istituto Nazionale di Fisica Nucleare, Sezione di Padova, I-35131 Padova, Italy
³ Centre for Astrophysics, Guangzhou University, Guangzhou 510006, People’s Republic of China
⁴ INAF – Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
⁵ Astronomy Science and Technology Research Laboratory of Department of Education of Guangdong Province, Guangzhou 510006, People’s Republic of China
⁶ Wuhu Vocational Technical University, School of Network Engineering, Anhui, People’s Republic of China
⁷ Institute of Astronomy and National Astronomical Observatory, Bulgarian Academy of Sciences, 72 Tsarigradsko shosse Blvd., 1784 Sofia, Bulgaria

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Received: 21 August 2025
Accepted: 10 February 2026

Abstract

Context. Based on the classical unification, blazars, namely BL Lacertae objects (BL Lacs) and flat-spectrum radio quasars (FSRQs), are believed to correspond to radio galaxies, when observed at small jet viewing angles.

Aims. In this paper, we aim to compile a sample of Fermi blazars [redshift, z ∈ (0.002 − 4.313)] and radio galaxies [z ∈ (0.001 − 1.048)] to provide new insights into a unified accretion and ejection scenario between aligned and misaligned radio-loud active galactic nuclei, by considering their optical emission-line classifications (low- and high-excitation radio galaxies, LERGs and HERGs, respectively), which are more representative of their accretion states.

Methods. We adopted the statistical analyses of accretion properties and high-energy beaming patterns for both Fermi blazars and radio galaxies to investigate a unified accretion–ejection scenario.

Results. In the γ-ray luminosity–photon index plane, HERGs populate the region of higher luminosities and softer photon indices, akin to FSRQs, whereas LERGs fill at lower luminosities with harder photon indices, analogous to BL Lacs. This parallel segregation indicates that LERGs and HERGs represent the misaligned counterparts of BL Lacs and FSRQs, respectively. The unified picture is further supported by the Compton dominance–photon index diagram, whereby FSRQs and HERGs dominated by external Compton (EC) emissions are distinctly separated from BL Lacs and LERGs governed by synchrotron self-Compton (SSC) emissions. Similarly, the diagram of accretion rate versus γ-ray photon index reveals two distinct accretion–ejection states: a low-accretion-rate branch (BL Lacs and LERGs) is associated with the SSC model, and a high-accretion-rate branch (FSRQs and HERGs) is linked to the EC model. These results strongly strengthen the idea of a unified accretion and ejection paradigm between blazars and radio galaxies separating into two distinct states.