Linking {\it Fermi} blazars and radio galaxies through accretion and jet radiation mechanisms

Abstract

Based on the classical unification, blazars, namely BL Lacertae objects (BL Lacs) and flat-spectrum radio quasars (FSRQs), are believed to correspond with radio galaxies when observed at small jet viewing angles. In this paper, we aim to compile a sample of Fermi blazars and radio galaxies to provide new insights towards a unified accretion and ejection scenario between aligned and misaligned radio-loud active galactic nuclei (AGNs), by considering their optical emission-line classifications (low- and high-excitation radio galaxies, LERGs, HERGs), which are more representative of their accretion states. We adopted statistical analyses of accretion properties and high-energy beaming patterns for both Fermi blazars and radio galaxies to investigate a unified accretion-ejection scenario. In the gamma-ray luminosity-photon index plane, HERGs populate the region of higher luminosities and softer photon indices, akin to FSRQs, whereas LERGs fill 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, where 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 gamma-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 into two distinct states.

11 pages, 7 figures, 4 tables, accepted by A&A

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