2017MNRAS.471.2848L

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 the bolometric Eddington ratio λ ∼ 10^–2. In this work, we strengthen this scenario through investigation of the relationship between the radio loudness R_UV and the optical-to-X-ray spectral index α_ox in LLAGNs with 10^- 6^ <= λ <= 10^- 3^. We compile from the literature a sample of 32 LLAGNs, consisting of 18 low-ionization nuclear emission-line region galaxies and 14 low-Eddington-ratio Seyfert galaxies, and observe a strong negative R_UV-α_ox relationship, with large scatter in both R_UV and α_ox. We further demonstrate that this negative correlation, and the additional two negative relationships reported in the literature (R_UV-λ and α_ox-λ correlations), can be understood consistently and comprehensively under the truncated accretion-jet model, a model that has been applied successfully applied to LLAGNs. We argue that the scatter in the observations is (mainly) due to the spread in the viscosity parameter α of a hot accretion flow, a parameter that can potentially serve as a diagnosis of the strength and/or configuration of magnetic fields in accretion flows.