A strong negative correlation between radio loudness $R_{\rm UV}$ and optical-to-X-ray spectral index $\alpha_{\rm ox}$ in low-luminosity AGNs

TL;DR

This study reveals a strong negative correlation between radio loudness and the optical-to-X-ray spectral index in low-luminosity AGNs, supporting the truncated accretion--jet model and highlighting the role of magnetic fields.

Contribution

It provides the first comprehensive analysis of the negative $R_{UV}$--$ m ox$ relationship in LLAGNs within the truncated accretion--jet framework, linking observational scatter to accretion flow parameters.

Findings

Strong negative $R_{UV}$--$ m ox$ correlation observed in LLAGNs.

Negative correlations between $R_{UV}$--$ m lambda$ and $ m ox$--$ m lambda$ confirmed.

Scatter explained by variations in the viscosity parameter $ m alpha$.

Abstract

It has been argued for years that the accretion mode changes from bright active galactic nuclei (AGNs) to low-luminosity AGNs (LLAGNs) at a rough dividing point of bolometric Eddington ratio $\lambda \sim 10^{-2}$. In this work, we strengthen this scenario through investigation of the relationship between the radio loudness $R_{\rm UV}$ and the optical-to-X-ray spectral index $\alpha_{\rm ox}$ in LLAGNs with $10^{-6} \lesssim \lambda \lesssim 10^{-3}$. We compile from literature a sample of 32 LLAGNs, consisting 18 LINERs and 14 low Eddington ratio Seyfert galaxies, and observe a strong negative $R_{\rm UV}$--$\alpha_{\rm ox}$ relationship, with large scatter in both $R_{\rm UV}$ and $\alpha_{\rm ox}$. We further demonstrate that this negative correlation, and the additional two negative relationships reported in literature ($R_{\rm UV}$--$\lambda$ and $\alpha_{\rm ox}$--$\lambda$ correlations), can be understood consistently and comprehensively under the truncated accretion--jet model, the model that has been applied successfully applied to LLAGNs. We argue that the scatter in the observations are (mainly) due to the spread in the viscosity parameter $\alpha$ of a hot accretion flow, a parameter that potentially can serve as a diagnose of the strength and/or configuration of magnetic fields in accretion flows.