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

TL;DR

The paper tests a two‑state unified accretion–ejection framework linking blazars and radio galaxies by assembling a large Fermi‑detected sample classified via emission‑line states (LERG/HERG) and comparing gamma‑ray properties ($L_{\gamma}$, $\Gamma_{\gamma}$), Compton dominance (CD), and accretion rates ($L_{\rm bol}/L_{\rm Edd}$). It finds that BL Lacs align with LERGs in SSC‑dominated jets, while FSRQs align with HERGs in EC‑dominated jets, with a clear separation in the $\Gamma_{\gamma}$–$L_{\gamma}$ and $\Gamma_{\gamma}$–CD planes; moreover, a transition around $\log(L_{\rm bol}/L_{\rm Edd}) \sim -2$ marks the shift from SSC to EC processes. The CD vs $\Gamma_{\gamma}$ analysis corroborates this unification, showing BL Lacs with low CD near the SSC locus and FSRQs with high CD near the EC locus, with LERGs and HERGs following analogous trends. The study also demonstrates that redshift biases do not drive the results and that FR 0s fit the LERG population, reinforcing a physically based unification that transcends extended radio morphology. Overall, the work advances a cohesive picture in which two accretion–ejection states govern the high‑energy emission of both beamed and misaligned RLAGNs, grounded in the SSC/EC dichotomy and accretion‑rate regimes.

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.

Figures (7)

• Figure 1: The plot of the relation between the $\gamma$-ray luminosities ($L_{\gamma}$) and $\gamma$-ray photon indices ($\Gamma_{\gamma}$). The green triangle is for LERGs, the green square is for FR 0 LERGs, and the black pentagram is for HERGs. BL Lacs are in red diamonds, while FSRQs are in blue circles.
• Figure 2: BPT diagnostic diagram showing the emission-line ratios $\log([\mathrm{N\,II}]/\mathrm{H}\alpha)$ versus $\log([\mathrm{O\,III}]/\mathrm{H}\beta)$ for the SDSS-selected sample. The demarcation curves from kauffmann03apj, kewley2001apj, and cid10mnras are shown to separate star-forming, composite, LERG, and HERG regimes. Sources located below the orange dashed–dotted line are classified as LERGs (green triangles), while those above are classified as HERGs (black pentagons).
• Figure 3: The distribution of the Compton dominances (CD) for blazars (upper panel) and radio galaxies (lower panel). Both of them scale into the same bin.
• Figure 4: The plot of the relation between the Compton dominance (CD) and the $\gamma$-ray photon indices ($\Gamma_{\gamma}$). The labels are the same as Fig. \ref{['lum-index']}. The red solid line is the linear regression for BL Lacs [$\Gamma_{\gamma} =(0.27 \pm 0.03)\,\log \mathrm{CD} + (2.13 \pm 0.02)$], and the blue dashed line is the linear regression for FSRQs [$\Gamma_{\gamma}=(0.08 \pm 0.02)\,\log \mathrm{CD} + (2.43 \pm 0.01)$].
• Figure 5: The distribution of the accretion rates ($L_{\rm{bol}}/L_{\rm{Edd}}$) for blazars (upper panel) and radio galaxies (lower panel). Both of them scale into the same bin.