A strong negative correlation between radio loudness RUV and optical-to-X-ray spectral index αox in low-luminosity AGNs

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 lambda similar to 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 alpha(ox) in LLAGNs with 10(-6) less than or similar to lambda less than or similar to 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-Eddingtonratio Seyfert galaxies, and observe a strong negative R-UV-alpha(ox) relationship, with large scatter in both R-UV and alpha(ox). We further demonstrate that this negative correlation, and the additional two negative relationships reported in the literature (R-UV-lambda and alpha(ox)-lambda 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 alpha 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.